Aggregor is an evil osmosian. He was main villian of first season Ben 10 Ultimate Alien. He is over 100 year olde. He looks like a human except for 4 antlers on his forehead. He carried a metal shaft that was able to fire energy blast. He has his own army of robots.
He was first seen abducting five powerful aliens from Andomeda Galaxy, Bivalvan, Galapagus, P'andor, Ra'ad and Andreas. His plan was to absorb their powers using a machine(so he can get 100% of their power instead of 10% he gets when he uses his power to absorb them.) and become powerful enough to get to Forge of Creation.
Aggregor, in his true form has enhanced strength and durability.
He absorbed all five aliens to gain a new appearance and became very strong and powerful. He had Andreas' shockwaves, Bivalvan's hydrokinesis and invulnerabilty., Ra'ad's electrokinesis and mind reading, Galapagus' immunity to magic and mana and Pandor's nuclear energy powers.
After gaining this form, he started collecting all four parts of map pf infinity. After taking all of them, he managed to go to forge of creating, where he planned to absorb a baby celestialsapien, so he could have omnipotence without their split personality. Ben from present and past tried to stop him, but both of them were easily defeated. So was Gwen. Kevin absorbed Ultimatrix and easily defeated Ultimate Aggregor by absorbing his powers.