How do I handle a character bent on diablerie of an Antediluvian?

I’m running a campaign and a character decided to change his goal from getting gun heels that are possessed by a demon (Bayonetta) to becoming third generation through diablerie. He’s made a deal with said demon in his heels (before the demon went in the heels) to become uneclipseable but I know that if at any point he wakes an Antediluvian the entire world will be screwed. I’m slowly “feeding” him elders, and I’m dreading when I have to bring an Antediluvian in. I’m almost certain that if I do throw an Antediluvian at him it’ll be the Ravnos Antediluvian as he’s bound to wake soon anyway.

How do I not kill the entire chantry with an Antediluvian?