Boundless Necromancer - Chapter 230

As a promise, he went out into the world as an assassin. 3 years later, after becoming the Supreme Assassin, he gave up on everything he had and returned to the ranks of ordinary people. But one day, Kismet, a mystical game created by the National Research and Development had appeared.

Enigmatic hands, spectral frame, and one cunning strike just like the swing of the Grim Reaper. The Supreme Assassin takes on the mystical world, and how his legendary tales sets off, progressively ascending the altar to becoming the best.

- Description from Novelupdates

Reborn in World of Warcraft : Mastering All Beauty

After dying from betrayal, Thomas reincarnates as Jolthar Kaelzhar, the neglected son of a powerful clan, just after his brothers kill him. With Jolthar's memories and the blessing of Qalena, Thomas vows to rise from his lowly position and claim power in his new life.

Living things really are ignorant – the greed to obtain the key to knowledge and the fundamentals of energy overpowers them. Little do they know that knowledge is full of lies and energy is the root of destruction. Us, mages, have mastered travelling through space and time. We are invincible, we can turn worlds into dust and you, mere ants, will be wiped out with no trace.

-Mage Adam

An A.I.(Artificial Intelligence) gets reincarnated into a world where he becomes a mage.