Malfegor

Malfegor was a rare hybrid beast, a dragon with a demonic soul, from the shard of Alara known as Grixis.

Before the Sundering
He was ancient, having existed before the Sundering of Alara. During this time, he slew the archangel Asha and was thought to be killed by many on Bant. However, he had in fact, just been exiled to Grixis.

On Grixis
On Grixis, he rose to immense power, becoming one of the most feared creatures on that world as well. He built a prison out of the bones of his victims. At one point, Malfegor would kidnap fledgling planeswalkers, and torture them in the hopes that they would be forced to planeswalk to avoid death. His first success was Sifa Grent. He held her prisoner, trying to understand the magic that allowed her to traverse worlds.

When Dack Fayden came to Grixis, he was quickly overpowered and thrown into a cell. There he was threatened with being eaten by Malfegor, but when he escaped he learned of Malfegor's experiments with fledgling planeswalkers. After Dack killed Malfegor's head jailer, Malfegor tried to take matters into his own hands but Dack escaped.

In league with Bolas
Sometime after the Mending, the elder dragon Nicol Bolas contacted Malfegor and persuaded him to prepare an army to invade the other shards after the coming Conflux. He performed his duties admirably, leading a troop across the shard of Esper, wiping out nearly every sphinx except Sharuum, before reaching Bant and staging an assault in an effort to activate the Obelisk of Bant that Bolas needed to complete his plan.

During this siege of Bant, Malfegor was killed by the combined forces of Rafiq and Elspeth Tirel.