Mjarin had a hand in killing Leitrum (Emperor) of the Cath Bruig Empire and proclaimed himself Emperor. He was eventually beheaded by Connacht.
What became of Mjarin after the Great War is a mystery. However, the final ending to Myth 2 Soulblighter suggests that the cycle has been broken. Given that the Leveler was in possession of Mjarin and Balor--and both wanted to destroy the Light--then perhaps the Leveler is still on the planet. If Mjarin's head could return to his body or Balor's body...who knows what would happen.
Disputed Canon Edit
It is often considered by many Myth fans that Mjarin was not the Leveler, however this is a subject of conjecture due to Moagim being the Leveler to end the Age of Reason. However, Moagim is known as the "Faceless Terror," it is plausible that the being that was posing as Moagrim was actually the Faceless Man, a Fallen Lord that would return during the Sword Age. This would make sense as the Leveler would want all the heroes for Light and Dark to fall under his banner, and this would prove Mjarin as the Leveler, it would keep Moagim dead, and Connacht would still be considered the great hero of the Wind/Wolf Age. It is totally canonical and should be considered the most logical sequence of events.