Leareth doesn't say anything else. He keeps walking. He thinks.
...There are a lot of things he should have checked for himself, before. But he didn't. He's here, now, Melkor captured him personally, and - maybe it's true, their awful story, but maybe it isn't, and he wouldn't know. He's under their power now.
If it is true, then he's yet again been made a pawn in a god's scheme – bringing Vanyel just in time for him to die and take out half a continent with him.
(Urtho...)
But he's not sure that there's any way of verifying their story that he would trust, at this point. Which means that if he lets himself be drawn into some scheme, it's as likely as not to be a trick that only causes harm.
It hurts, to give up. To admit he's been backed into a corner. It hurts less than he would have expected. Knowing that Vanyel is dead, and thanks to his own mistake, is worse.
But there are other worlds. He can't be the only person in a whole multiverse who's actually trying. And he's not about to be responsible for unleashing an evil god on it. Maybe Melkor isn't evil but that's not a gamble he can take. Maybe all of it is a lie and Vanyel is alive and they'll still be able to win. Either way, he's pretty sure he knows what option here is best in expectation for his goals.
Maybe his immortality path is blocked and maybe it isn't. No way to confirm yes or no that won't give everything away.
He doesn't want to die. There's a screaming pit of no. But it wouldn't really be him who survived, if he were to gamble wrong and end up on a path that makes the world worse rather than better. Not worth it.
"Maybe," he says, calm, unruffled. "Maybe not." I am not going to help you, but he is absolutely not going to give them any more leverage by saying why.
I am sorry, Vanyel. Words he won't ever have the chance to say, any more than he said them to Urtho.
It's not the first time Leareth has called down a Final Strike. He's probably the only person ever who is experienced at it - some quiet part of him is expecting it not to work, but he reaches for it anyway...