That Good Shiro/Lotor content

So this is PURELY self-indulgent and not at all meant to be anything close to canon but!
Imagine that as Zarkon's son, Lotor was groomed to be the next successor of the Black Lion and began attempting to forge a connection with her since he was very young (though that bond was impeded by Zarkon, who didn't want any competition, obviously.) But after Shiro takes over the Black Lion, where he progresses faster in bonding with his lion than any of the other paladins because he's a true natural, he starts experiencing these... visions.
Part of paladin training requires mindfulness and meditation, and when deep into meditation, Shiro finds that he is able to connect to his lion and slip into a strange space, separate from the astral plane, but contained within it at the same time, a tiny pocket of universe all his own. But it isn't all his own.
And he isn't alone.
A surprised but fascinated Lotor is there to greet him and asks him how he was able to barge in on Lotor practicing his own form of meditation (but entirely different from Shiro's sitting and breathing, Lotor meditates through movement and finds he's able to rid himself of all distractions when fighting.)
They talk, Shiro claims his title as the Black Paladin (one that feels heavy on his tongue; it still doesn't feel real to him yet) and Lotor, amused, says that there can be no other Black Paladins since Voltron was lost centuries ago. Lotor doesn't look like a typical Galra but Shiro still feels something is wrong and looks for a way out only to find that there is none. Just a blank, dark space with stars pulsing through it.
"Having trouble?" Lotor asks. "You really must be new to this after all." And as soon as he finishes speaking, a sword that looks much like the weapon Keith is able to summon from his bayard appears, glowing deep purple, right before it buries itself into Shiro's chest.
"I'll help you out just this once."
And he wakes up.
Shiro avoids his training for a while, pretends that he's trying his hardest but can't quite grasp that same ease he showed at first, all because he fears that Lotor will be waiting with another sword when he enters that space again, but it's impeding the group's progress and he can't avoid it forever, so once again, there he is, and Lotor isn't there. It's just him and it's calm and Shiro finally feels he can breathe. He can stop worrying. His fledgling team grows closer, but eventually, he runs out of luck and there Lotor is again during the few hours of sleep he can manage to catch in between drills and nightmares. He thinks this is another nightmare come to life.
"Oh, there you are. Finally." Calm as you'd ever see, grinning like they were old friends catching up over coffee, though his coffee dates had never shown so much fang. "Sorry about that little unpleasantness earlier, I was hoping to catch you before now, but you've been elusive. Not avoiding me, I hope?"
Lotor explains that in this space, built from their shared bond with the Black Lion, with a little effort and practice, you can will pretty much anything into existence, within reason. Leaving can be a little tricky until you've got a good handle on how it all works, and one thing guaranteed to sever that connection is causing a big enough shock that your mind snaps back to reality, hence: Swords.
"You might have wanted to explain that before you tried running me through with a dream sword."
"Oh, quit being a baby, it's not like it was real."
One thing does immediately occur to Shiro, though.
If this is made from having a bond with the Black Lion, then why is Lotor here? Sensing a theory that fills him with excitement, he asks if Lotor is another Black Paladin, one of those "Paladins of Old" that Allura's always raving about.
Lotor's smile loses that catlike edge to it, and he flaps a hand dismissively, "Oh, god, no, not on your life. Maybe once, but you look like you've got the position well filled."
He takes a second to look Shiro up and down, sizing him up and clucking his tongue at what he finds. "If this is all the head of Voltron amounts to, I pity the universe. Are you even any good in a fight?" Shiro bristles at that, shoots back that he's good enough, and Lotor's grin is back full force as he offers to test that, summoning (what else?) yet another sword.
Things continue on like that for some time, with Lotor testing every edge of his defenses, bemoaning his offense, taunting him, goading him, and providing running commentary with each swing and block. They keep going before Shiro realizes he's not tired, not panting, sweating, or slowing down from fatigue. "Convenient, isn't it? I've obviously never gotten the chance to spar here, all by my lonesome, but this just might work." He disengages, the sword disappears, and Lotor finally nods and says, "You'll do."
From then on. that's their routine. Small-talk where Shiro tries prying for Lotor's identity and Lotor probes for intel on Shiro's comrades or location before they get to sparring until Shiro is inevitably jostled, the alarm for a Robeast attack sounds, or his concentration otherwise breaks. Sometimes, the talking goes on for so long that their sparring time is cut short, but it ends up in visible improvement on Shiro's end as he picks up these calculated, foreign, alien techniques that nobody recognizes.
Allura keeps quiet about her suspicions, but she always watches closely.
It starts on a routine mission destroying a small fleet on the outskirts of the Galra empire, with a voice that he could recognize (literally) in his sleep, saying only, "Don't disappoint me now. Let's make this a fair chase. You'll hide and I'll seek."
Shiro gets the sinking feeling in his stomach as his eyes confirm what he'd already known but didn't want to accept as Lotor walks out, flanked by two of his guards that he can immediately match names to their faces from Lotor's descriptions alone, full of fond exasperation that he was beginning to glimpse when he caught Lotor looking at him.
Shiro knows what he has to do, what he should be doing, is going after Lotor with everything he has; he can't disappoint his team, can't let them down, can't fail them as a leader, but his legs won't move. His arm fails to activate. He's shaking almost imperceptably as Lotor strides over, casual as could be, while the other paladins lift their weapons with every intent to defend their leader. What's wrong with him? Why isn't he moving? Why won't he protect himself?
Lotor's too well-bred to snort, but he does laugh at Keith barking an order to stay back and Lance hefting his rifle, Pidge's katar crackling with electricity, Hunk moving to put himself between them, and lays a careful, possessive, familiar hand underneath Shiro's chin.
"That's not any way to greet a friend. I've come so far to say hello. Why haven't you introduced us, Shirogane?"