That Time I Got Reincarnated as a Slime: Clayman's Revenge

Demon Lord Clayman was killed by Rimuru at Walpurgis. But, Clayman acquired the unique skill "Regressor" while on the verge of death and woke up in his room 70 years ago. Considering the situation, it appears that this it's actually real. Clayman begins to look for ways to avoid the same mistakes he