The Academy's Immortal Professor
D25/100Web serial by Evil Lemon
After a lifetime as humanity's strongest, Lucian never expected to become a professor. Once he defeated the Demon Lord, he disappeared to relax for a few centuries — and only came back when an old friend called in a favor. That friend also happened to see the future, and apparently a whole string of disasters was on the way, all somehow tied to a certain academy. Naturally, Lucian was suspicious when the solution turned out to be a teaching job, but he was willing to give it a shot. All he had to deal with were scheming nobles, mysterious threats lurking beneath the surface, a few old enemies who really should have stayed dead, and a new enigmatic System to worry about. Still, Lucian knew he could handle anything. Especially right after a good sandwich.