Flagg powers

Death Manipulation
He possesses a minor aura of death. Tom Cullen describes it while under hypnosis.
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He looks like anybody you see on the street. But when he grins, birds fall dead off telephone lines. When he looks at you a certain way, your prostate goes bad and your urine burns. The grass yellows up and dies where he spits.
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We later see that what Tom said was not hyperbolic. During a point where he's suffering from amnesia, Flagg's mere presence kills things.
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He turned around. Green jungle seemed to leap out at his eyes, a dark forested tangle of vines and broad leaves and lush, blooming flowers that were (as pink as a chorus girl’s nipple)
He was bewildered again.
What was a chorus girl?
For that matter, what was a nipple?
A macaw screamed at the sight of him, flew away blindly, crashed into the thick bole of an old banyan tree, and fell dead at the foot of it with its legs sticking up.
(sat him on the table with his legs stickin up)
A mongoose looked at his flushed, beard-scruffy face and died of a brain embolism.
(in come sis with a spoon and a glass)
A beetle that had been trundling busily up the trunk of a nipa palm turned black and shriveled to a husk with tiny blue bolts of electricity frizzing for a moment between its antennae.
(and starts dippin gravy from its yass-yass-yass.)
Who am I?
He didn’t know.
Where am I?
What did it matter?
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A nightmare causes this aura to activate. It kills a nearby guard and causes another to go blind.
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Both of the above scenes imply that he has to keep control over this aura himself.

In the Dark Tower film (which is a canon sequel to the novels), he shows that he can focus these death-infducing effects through his commands. Saying the word "Goodbye" instantly kills a person.
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Subconscious morality/willpower/empathic manipulation:
Turns a hostile tribe of twelve into his slaves with nothing but a smile
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He began to walk—stagger—toward the verge of the jungle. He was light-headed with hunger. The sound of the surf boomed hollowly in his ears like the beat of crazy blood. His mind was as empty as the mind of a newborn child.
He was halfway to the edge of the deep green when it parted and three men came out. Then four. Then there were half a dozen.
They were brown, smooth-skinned folk.
They stared at him.
He stared back.
Things began to come.
The six men became eight. The eight became a dozen. They all held spears. They began to raise them threateningly. The man with the beard-stubble on his face looked at them. He was wearing jeans and old sprung cowboy boots; nothing else. His upper body was as white as the belly of a carp and dreadfully wasted.
The spears came all the way up. Then one of the brown men—the leader——choked out one word over and over again, a word that sounded like Yun-nah!
Yep, things were coming.
Righty-O.
His name, for one thing.
He smiled.
That smile was like a red sun breaking through a black cloud. It exposed bright white teeth and amazing blazing eyes. He turned his lineless palms out to face them in the universal gesture of peace.
Before the force of that grin they were lost. The spears fell to the sand; one of them struck point-down and hung there at an angle, quivering.
“Do you speak English?”
They only looked.
“Habla español?”
No they didn’t. They definitely did not habla fucking espanol.
What did that mean?
Where was he?
Well, it would come in time. Rome wasn’t built in a day, nor Akron, Ohio, for that matter. And the place didn’t matter.
The place where you made your stand never mattered. Only that you were there ... and still on your feet.
“Parlez-vous français?”
No answer. They stared at him, fascinated.
He tried them in German, and then bellowed laughter at their stupid, sheepy faces. One of them began to sob helplessly, like a child.
They are simple folk. Primitive; simple; unlettered. But I can use them. Yes, I can use them perfectly well.
He advanced toward them, lineless palms still turned outward, still smiling. His eyes sparkled with warm and lunatic joy.
“My name is Russell Faraday,” he said in a slow, clear voice. “I have a mission.”
They stared at him, all eyes, all dismay, all fascination.
“I have come to help you.”
They began to drop on their knees and bow their heads before him, and as his dark, dark shadow fell among them, his grin widened.
“I’ve come to teach you how to be civilized!”
“Yun-nah!” the chief sobbed in joy and terror. And as he kissed Russell Faraday’s feet, the dark man began to laugh. He laughed and laughed and laughed.
Life was such a wheel that no man could stand upon it for long.
And it always, at the end, came round to the same place again.
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Skin contact causes Andrew Quick/The Tick-Tock Man's crippling fear of him to disappear immediately
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“Call me Fannin,” the grinning apparition said. “Richard Fannin. That’s not exactly right, maybe, but I reckon it’s close enough for government work.” He held out a hand whose palm was utterly devoid of lines. “What do you say, pard? Shake the hand that shook the world.”
The creature who had once been Andrew Quick and who had been known in the halls of the Grays as the Tick-Tock Man shrieked and again tried to wriggle backward. The flap of scalp peeled loose by the low-caliber bullet which had only grooved his skull instead of penetrating it swung back and forth; the long strands of gray-blonde hair continued to tickle against his cheek. Quick, however, no longer felt it. He had even forgotten the ache in his skull and the throb from the socket where his left eye had been. His entire consciousness had fused into one thought: I must get away from this beast that looks like a man.
But when the stranger seized his right hand and shook it, that thought passed like a dream on waking. The scream which had been locked in Quick’s breast escaped his lips in a lover’s sigh. He stared dumbly up at the grinning newcomer. The loose flap of his scalp swung and dangled.
“Is that bothering you? It must be. Here!” Fannin seized the hanging flap and ripped it briskly off Quick’s head, revealing a bleary swatch of skull. There was a noise like heavy cloth tearing. Quick shrieked.
“There, there, it only hurts for a second.” The man was now squatting on his hunkers before Quick and speaking as an indulgent parent might speak to a child with a splinter in his finger. “Isn’t that so?”
“Y-Y-Yes,” Quick muttered. And it was. Already the pain was fading. And when Fannin reached toward him again, caressing the left side of his face, Quick’s jerk backward was only a reflex, quickly mastered. As the lineless hand stroked, he felt strength flowing back into him. He looked up at the newcomer with dumb gratitude, lips quivering.
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High resistance to poisons and chemicals:
States that Blaine The Mono's biological weapons will have no effect on him.
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“Thank you, Andrew,” the dark man said softly. “Now we must step lively—I’m expecting a drastic change in the atmosphere of these environs in the next five minutes or so. We must get to the nearest closet where gas masks are stored before that happens, and it’s apt to be a near thing. I could survive the change quite nicely, but I’m afraid you might have some difficulties.”
“I don’t understand what you’re talking about,” Andrew Quick said. His head had begun to throb again, and his mind was whirling.
“Nor do you need to,” the stranger said smoothly. “Come, Andrew— I think we should hurry. Busy, busy day, eh? With luck, Blaine will fry them right on the platform, where they are no doubt still standing—he’s become very eccentric over the years, poor fellow. But I think we should hurry, just the same.”
He slid his arm over Quick’s shoulders and, giggling, led him through the hatchway Roland and Jake had used only a few minutes before.
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This is what those chemicals/toxins/etc. do to people.
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“BLAINE, WHAT’S THAT?” JAKE asked, but he already knew.
Blaine laughed . . . but made no other reply.
The purple vapor drifted from gratings in the sidewalk and the smashed windows of deserted buildings, but most of it seemed to be coming from manholes like the one Gasher had used to get into the tunnels below the streets. Their iron covers had been blown clear by the explosion they had felt as they were boarding the mono. They watched in silent horror as the bruise-colored gas crept down the avenues and spread into the debris-littered side-streets. It drove those inhabitants of Lud still interested in survival before it like cattle. Most were Pubes, judging from their scarves, but Jake could see a few splashes of bright yellow, as well. Old animosities had been forgotten now that the end was finally upon them.
The purple cloud began to catch up with the stragglers—mostly old people who were unable to run. They fell down, clawing at their throats and screaming soundlessly, the instant the gas touched them. Jake saw an agonized face staring up at him in disbelief as they passed over, saw the eyesockets suddenly fill up with blood, and closed his eyes.
Ahead, the monorail track disappeared into the oncoming purple fog. Eddie winced and held his breath as they plunged in, but of course it parted around them, and no whiff of the death engulfing the city came to them. Looking into the streets below was like looking through a stained-glass window into hell.
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Should also be noted that he was one of the few people on the planet (as per the story of The Stand) who was completely immune to the Captain Trips superflu, as he was one of the survivors.

Implied resistance to mind manipulation:
Regularly uses Maerlyn's Grapefruit, as it belongs to the Crimson King and was created by Maerlyn, his (Flagg's) father. It mindhaxes its users, like so.
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It can read the minds of its users
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The scan above also makes it clear that the Grapefruit cannot read Flagg's mind; it only finds out that Flagg has been seeing Roland's mother by way of reading Steven Deschain's mind, heavily implying that Flagg himself was able to shield his memories from her for all this time.

Eldred Jonas touched it barehanded exactly one time in the series, and it affected him immediately.
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He held his hands out patiently, saying nothing, waiting for her mind to accept reality—if she let go, there was some chance. If she held on, very likely everyone in this stony, weedy yard would end up riding the handsome before long.
With a sigh of regret, she finally put the ball in his hands. At the instant it passed from her to him, an ember of pink light pulsed deep in the depths of the glass. A throb of pain drove into Jonas’s head . . . and a shiver of lust coiled in his balls.
As from a great distance, he heard Depape and Reynolds cocking their pistols.
“Put those away,” Jonas said.
“But—” Reynolds looked confused.
“They thought’ee was going to double-cross Rhea,” the old woman said, cackling. “Good thing ye’re in charge rather than them, Jonas . . . mayhap you know summat they don’t.”
He knew something, all right—how dangerous the smooth, glassy thing in his hands was. It could take him in a blink, if it wanted. And in a month, he would be like the witch: scrawny, raddled with sores, and too obsessed to know or care.
“Put them away!” he shouted.
Reynolds and Depape exchanged a glance, then reholstered their guns.
“There was a bag for this thing,” Jonas said. “A drawstring bag laid inside the box. Get it.”
“Aye,” Rhea said, grinning unpleasantly at him. “But it won’t keep the ball from takin ye if it wants to. Ye needn’t think it will.” She surveyed the other two, and her eye fixed on Reynolds. “There’s a cart in my shed, and a pair of good gray goats to pull it.” She spoke to Reynolds, but her eyes kept turning back to the ball, Jonas noticed . . . and now his damned eyes wanted to go there, too.
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And afterwards he went from this:
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The ball was out of its bag and lay in Rhea’s lap. “Anything?” he asked. He both hoped and dreaded to see that deep pink pulse inside it again.
“Nay. It’ll speak when it needs to, though—count on it.”
“Then what good are you, old woman?”
“Ye’ll know when the time comes,” Rhea said, looking at him with arrogance (and some fear as well, he was happy to see).
Jonas spurred his horse back to the head of the little column. He had decided to take the ball from Rhea at the slightest sign of trouble. In truth, it had already inserted its strange, addicting sweetness into his head; he thought about that single pink pulse of light he’d seen far too much.
Balls, he told himself. Battlesweat’s all I’ve got. Once this business is over, I’ll be my old self again.
Nice if true, but . . .
. . . but he had, in truth, begun to wonder.
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To this:
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He grabbed the bag just below the draw top and yanked. Rhea screamed again as the string skinned her knuckles and tore off one of her nails. Jonas hardly heard. His mind was a white explosion of exultation. For the first time in his long professional life he forgot his job, his surroundings, and the six thousand things that could get him killed on any day. He had it; he had it; by all the graves of all the gods, he had the fucking thing!
Mine! he thought, and that was all. He somehow restrained the urge to open the bag and stick his head inside it, like a horse sticking its head into a bag of oats, and looped the drawstring over the pommel of his saddle twice instead. He took in a breath as deep as his lungs would allow, then expelled it. Better. A little.
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Rhea climbed back up, flopped onto the cantboard again with all the grace of a dying fish, and peered around at them, wall-eyed and sneering.
“I curse ye all!” she screamed. It cut through them, stilling their laughter even as the cart bounced toward the edge of the trampled clearing. “Every last one of ye! Ye . . . and ye . . . and ye!” Her crooked finger pointed last at Jonas. “Thief! Miserable thief!”
As though it was yours, Jonas marveled (although “Mine!” was the first word to occur to him, once he had taken possession of it). As though such a wonder could ever belong to a back-country reader of rooster-guts such as you.
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“Never mind!” Jonas shouted, pulling their attention back to him. He reached out a stealthy hand and caressed the curve at the bottom of the drawstring bag. Just touching the ball made him feel as if he could do anything, and with one hand tied behind his back, at that.
“Never mind her, and never mind them!” His eyes moved from Lengyll to Wertner to Croydon to Brian Hookey to Roy Depape. “We’re close to forty men, going to join another hundred and fifty. They’re three, and not one a day over sixteen. Are you afraid of three little boys?”
“No!” they cried.
“If we run on em, my cullies, what will we do?”
“KILL THEM!” The shout so loud that it sent rooks rising up into the morning sun, cawing their displeasure as they commenced the hunt for more peaceful surroundings.
Jonas was satisfied. His hand was still on the sweet curve of the ball, and he could feel it pouring strength into him. Pink strength, he thought, and grinned.
“Come on, boys. I want those tankers in the woods west of Eyebolt before the home folks light their Reap-Night Bonfire.”
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It can suck the minds out of people who anger it, trapping them inside of itself.
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Not only can Flagg touch the Grapefruit without ill effects (even after he has managed to piss it off in-story), but he proceeds to seal the demon back inside of it with no trouble whatsoever. Said demon makes it quite clear here that this was done against its will
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During his infamous final moments with Mordred, he shows that he is capable of sensing when people have breached his defenses and are reading his mind.
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“You may wonder why I’m here, and not about your father’s business,” Walter said. “Do you?”
Mordred didn’t, but he nodded, just the same. His stomach rumbled.
“In truth, I am about his business,” Walter said, and gave his most charming smile (spoiled somewhat by the peanut butter on his teeth). He had once probably known that any statement beginning with the words In truth is almost always a lie. No more. Too old to know. Too vain to know. Too stupid to remember. But he was wary, all the same. He could feel the child’s force. In his head? Rummaging around in his head? Surely not. The thing trapped in the baby’s body was powerful, but surely not that powerful.
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(Important note here: His general overconfidence here comes from the fact that he's currently wearing a "thinking cap" which is designed to block telepathy and psychic attacks. He placed his trust in this thing because, among other showings, it was capable of blocking his own telepathic powers when someone else was wearing it)
When he realizes he's being read, his thoughts very much imply that he has his own innate defenses that were breached in addition to the thinking cap, as he compares it to having someone break into his house.
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There’s a phrase, the elephant in the living room, which purports to describe what it’s like to live with a drug addict, an alcoholic, an abuser. People outside such relationships will sometimes ask, “How could you let such a business go on for so many years? Didn’t you see the elephant in the living room?” And it’s so hard for anyone living in a more normal situation to understand the answer that comes closest to the truth: “I’m sorry, but it was there when I moved in. I didn’t know it was an elephant ; I thought it was part of the furniture.” There comes an aha -moment for some folks—the lucky ones—when they suddenly recognize the difference. And that moment came for Walter. It came too late, but not by much.
Y’won’t shit on me, will you—that was the question he asked, but between the word shit and the phrase on me, he suddenly realized there was an intruder in his house … and it had been there all along. Not a baby, either; this was a gangling, slope-headed adolescent with pockmarked skin and dully curious eyes. It was perhaps the best, truest visualization Walter could have made for Mordred Deschain as he at that moment existed: a teenage housebreaker, probably high on some aerosol cleaning product.
And he had been there all the time! God, how could he not have known? The housebreaker hadn’t even been hiding! He had been right out in the open, standing there against the wall, gape-mouthed and taking it all in.
His plans for bringing Mordred with him—of using him to end Roland’s life (if the guards at the devar-toi couldn’t do it first, that was), then killing the little bastard and taking his valuable left foot—collapsed in an instant. In the next one a new plan arose, and it was simplicity itself. Mustn’t let him see that I know. One shot, that’s all I can risk, and only because I must risk it. Then I run. If he’s dead, fine. If not, perhaps he’ll starve before—
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