Dartag the Hatred

Dartag is the Ruby Dragon of Tiamat, A Great Wyrm and the first and only Ruby Dragon. His breath leveled entire Giant Fortresses in the old days of the War of the Ancients. He is a terror to behold, feared even by his queen for if he turned on her she would not be certain to survive. 
Dartag or as the giants reffered to him, The Hatred, is the largest of all Great Wyrms ever recorded, nearly as large as the mountains at the Northern Edge of the World. He is capable of reducing his size on a whim to engage more nimble opponents and his strength is legendary. His breath is not simply flame but mixed in are the magics of death causing those burned by him to suffer even further by tainting the wounds with foul energies. 
He has not been seen since the end of the War of the Ancients, thankfully for no force could stop him, and it is theorized he has been placed into a deep slumber by Tiamat herself, so that she could recover her strength after her battle with Bahamut. There are few places such a beast could reside and the last few bode poorly. It is theorized he is beneath the sea of Fire on the Elemental Plane of Fire or that he dwells within the Abyss, though none look to see for they would be terrified to awaken him.