Mephisto is one of the three Prime Evils, and one of the three who have meddled the most in Sanctuary's and humans' business. Mostly just to steal the energy of our mortal souls.
He has not only been part in the destruction and corruption of humans, he also had a hand in the very preservation of Sanctuary. His own daughter was also one of the rogue demons and angels to set about the creation of this new world...
Who would have thought he'd make a deal with Tyrael anyway?
This malicious and undead-loving Prime Evil is likely to return in Diablo III, but in the meanwhile, why not read up on his background?
Read more about the horrid Lord of Hatred on the Mephisto article.