My Covenant with the Monster Lord

Loyal king of demons said, "Now that you've found me, you need to be responsible for me." Lost memories and destined reunion. Yu Yin reincarnated twice and returned to the world he was originally from, but he got "scammed" by some unknown demon king the moment he's back. Moreover, he got bitten by him and formed an agreement! While Yu Yin was in astonishment, the demon king considered everything just normal. "I surely need to be responsible for you, given the relationship between us."

(Source: MangaToon)