Ok, OC idea, and pardon me if it’s been done before. The concept is an eldritch entity that meets the Doctor and makes a deal with them: if the Doctor can show them something new—something that an all-seeing, universe-spanning being like them has never experienced—they’ll let the Doctor live. The Doctor has a set amount of time to do it. The entity agrees, but to travel safely they must take on a disguise, something mortal enough not to drive others insane by existing. Part of their pact is that they can’t shed this disguise until either they witness something truly new or the Doctor fails to provide it.
So the Doctor takes them along on a typical adventure. There’s danger, chaos, the usual. And somewhere in the midst of it, the entity—trapped in this fragile, mortal form—feels something they’ve never known before: fear. They love it. They need another taste. They keep traveling with the Doctor, quickly becoming addicted to the rush of fear and adrenaline that comes from being breakable. (If you’re picturing them grinning with wild delight while about to die, you’re picturing them correctly.)
Over time, they start to feel… something else. Love? No, that’s not quite right. But there’s a spark of admiration, an affection they can’t articulate, and it terrifies them. That fear only makes it worse, and they savor every second of it. They can’t admit what’s happening, so they keep pretending—offering the Doctor “one more chance” to show them something new. Both of them eventually see through the ruse, but neither stops. They’re both too amused by the game.
But the longer the entity stays mortal, the more it unravels them. They begin to remember what they were, and the more they cling to this fragile existence, the more their mind and body start to crack. They’re caught between two incompatible states—cosmic and human—and it’s driving them toward madness. Eventually, it becomes clear that if they stay like this much longer, they’ll lose themselves completely.
The Doctor chooses to end the pact, to save them. But by that point, the entity doesn’t want to be saved. They’d rather die as something that can feel than return to what they were. They beg the Doctor to end their life. Of course, the Doctor can’t. Instead, they surrender, admitting defeat in the deal. The entity sheds their mortal form and loses all the emotion they’d gained—but before leaving, they tell the Doctor that while they no longer feel what they once did, they remember that they felt something. And that, they decide, is something new.
Bonus idea: bring them back later when the Doctor needs a favor. They remember the Doctor not with love, but with curiosity.
I’d call them V̶̧̰̎a̸̲͈̎̀̏z̷͇̾t̶̼̜̆̒͊o̶̠̲͊l̷͍̠̟͘b̵̪̃͝ȟ̸̢̭͙̏̑—or Vaz for short.