The discarded one (Honkai Star rail x Crossover Union)

A long time ago there was a man who was told to had done the unthinkable. By reaching the status of being an immortal being. And now he regrets it, why so he can be with the one's who took him under there wing. But now he can't ever die, and his only goal in life is to find the one person who can end his life. In a fight of glory. And so he travels across the stars to find the one who can end his life, so he knows peace once and for all.