To bring her back, he must kill all the monsters roaming the dungeons. He has done it more than 300 times (turning back time over 300 times!) though I cannot remember the exact number.
For Irene, this is her second life, but for Michael, it is an endless loop.
Through all those attempts, his body absorbed dark magic because he lacked a purifier partner to heal him. Irene is the only one purifier in his life.
In other words, he would go berserk as the dark magic polluted his body for hundreds of years. Eventually, the monsters recognized him as their king, as the dark magic residing in his body is the same energy that flows through them. Whenever he unleashes his true power, comes into contact with a monster or loses his sanity, his eyes sometimes turn red. However, he can control this at will so that Irene is able to purify him.

can somebody spoil me why his eyes were red near the end?