I Have Become The Demonic Ancestor

A kind young man accidentally transmigrated into the body of the master of the villainous demonic sect, just as the master lost his powers due to the thousand-year limit. Facing betrayal by his own disciples and attacks by other righteous sects, he must complete the missions issued by the Absolute System in order to survive. But those missions — are all about doing good and being helpful!? (Source: Bilibili, translated)