Eldrantulku, the Oathbreaker, is one of the Overlords who ruled over all of Eberron during the Age of Demons. Eldrantulku was bound by the dragons and couatl after they jointly discovered the Draconic Prophecy, and the couatl sacrificed themselves to form the Silver Flame.
All of the Overlords embody mortal fears. Eldrantulku embodies our fear of betrayal, that those we trust will turn their backs on us or otherwise cause us harm. This has some overlap with Bel Shalor, the Shadow in the Flame, but Eldrantulku doesn't care about the intentions of the betrayer, where Bel Shalor thrives when somebody betrays their own principles, Eldrantulku thrives every time a person betrays someone else.
The prophetic thread that leads to the release of Eldrantulku was discovered long ago, which stated that the overlord would be free of it's bonds after the fall of a great empire. As of yet, it seems that none of the empires that have fallen were the one mentioned in the prophecy, and it's possible that Eldrantulku's prakhutu, Kashtarhak, the Voice of Chaos, had a hand in the fall of the giants of Xen'drik, the Dhakaani Empire, and possibly even the Last War.