Cairax can kill bodies he possesses

“So it’ll kill anyone who goes past your marks,” he said.
“Yeah. Probably.”
“How?”
“Well, you’ve seen the exes. He’s got a good four or five seconds before those bodies explode.” He stopped shaving and glanced over his shoulder. “You’ve fought Cairax, George. How much damage do you think he can fit into five seconds?”
Stealth’s head shifted inside her hood. “What is the other possible result of passing the wards?”
“He might try to possess whoever goes out there. But the odds of pulling it off with an unprepared body are next to nil. Really, it’s just another way he could kill people.”
…

“If it isn’t suitably prepared with the right sigils and agreements, a normal human body just can’t take the stress of demonic possession.”
“Yours did,” said St. George.
“Yeah, but mine was prepared, plus I had the safeties in the medallion. Anyone else would just burst like the exes. It’s like boiling a frog—you’ve got to go slow to even have a chance of it working.” The sorcerer gestured at himself. “Look how long it took me to work my way into Jarvis’s body. He’d need at least twice as much time.”

Excerpt From Ex-Communication, pg 254-255
Peter Clines
https://books.apple.com/us/book/ex-communication/id635706546