Yawgmoth

Yawgmoth The Ineffable

   Yawgmoth was (and to many hopefuls, including myself, is) a pre-mending planeswalker who also happened to be the guy behind the original Phyrexia. TL:DR, he was a god, who had a whole plane of existence to call his own. Said plane of existence quickly became known as the Phyrexian Empire, with the signature "thing" as hordes of bio-mechanical warriors, demons, and monstrosities (Think a more grimdark AdMech with some sorcery thrown in, and you'll have a general idea) He also tried to take over Dominaria a few times (I think it was three) and kept doing evil things until he was killed by Urza and an alliance of planeswalkers. This is a very simplified explanation but should suffice in a pinch.

The Fluff

   Yawgmoth did not start his life in MTG lore as the ultimate bad guy, rather he was simply known as Yawgmoth, the healer, who was exiled from Dominarian civilization (known as "The Thran" who were standard humans) for "tinkering" a bit too much with his patients, and his methods were a bit too unorthodox for comfort. He was a drifter for a while, continuing his work around goblin and viashino tribes, until he got called back to mainline civilization. 

The Crunch

   Yawgmoth doesn't have his own card, and even now nobody knows for sure what he looks like. The reason for this anonymity is simple: he would completely break the game if he was added. Remember, this guy is one of the planeswalkers from before the mending, and is just as powerful as that entails. The natural argument to this is "treat him like another Nicol Bolas, or the Eldrazi Titans," if you think this, you simply are not comprehending how powerful Yawgmoth is. 

(This article is currently under construction, any helpful edits would be welcome).