A/B/O isn't my favorite trope, but I can write it. Does the OP have a specific kind of A/O system in mind re: type of omega genitalia, mpreg, knotting? Is the lock timed b/c Nicky will go feral or be unable to stop himself from leaving and finding some D? What physically causes death?

Obvs OP has got some killer ideas and I could sink my teeth into some identity porn, h/c, and the easing of centuries of abject loneliness with found family.