Tiamat was an alien warrior trained extensively to defeat Messiah/S'Met'Kth.  The Messiah brought peace to the universe, one planet at a time, by visiting said planet and making everyone stunned with joy. Taking away all free will and leaving behind mindless shells of the beings that once were there. Mindless, the races then stagnated, evolution stopped and death awaited. A peaceful ending my Messiah's view. Tiamat was supposed to stop such an ending from coming to the entire universe.

He came to Earth to find out Landau, Lockman and Lake believed the Messiah to be a hero and Tiamat a villain. He had to face the company's "Mithras", two heroes chosen for this cause. They were Deadpool and Captain America and were easily defeated in single combat. However when the duo returned for a rematch, Tiamat lost sight of his mission. He focused solely on defeating them again and left the Messiah free to act. The Elders of his race realized something was wrong with their champion and transferred the armor to Deadpool. Which completed the mission by slaying the Messiah.