Immortality doubts [on hold]

I’m pretty new at DMing, if you guys could give me some hints about this I would appreciate. One of my players is a megalomaniac necromancer who wants to make every living creature immortal, he wants to end death. What kind of information or research about means could I give to him to make his quest more interesting? I thought of something like having to find the research or library of a lich or something, but that sounds to obvious.