The Forsaken

Jack dies in the real world. He wakes up as a knight in a world called 'Lux'. He realizes he's in an isekai and thinks this is a dream come true. He hopes for all the standard tropes and a standard adventure involved with high jinx where he'd face the demon king, only to find the demon king has already been slain by someone else. However, killing the demon king also requires you to absorb him into yourself. The previous hero wasn't strong enough to hold him back, causing The Forsaken, to be born and spread a world ending infection to the surrounding villages to destroy the capital kingdom of Lux.