Arceus is a Great Being who controls Pokemon. He was the driving force behind the Battle of Neo Z'Traa during the Uterio War. Near the end of the battle, he was sent to the past by Blackout. He has since been restored to our time when a beam of Antimatter struck the exact spot he was pulled through, to be met by Ynot and convinced to help in the battle against Omega Supreme. He survived that battle, and has popped up periodically, conversing with Michael Weston.
It is perhaps because of these conversations with Weston that he was recently captured by DU Monarch and placed in the UG.