Zumaki

Zumaki of the Bottomless Pool was an old dragon, who had gathered enough treasure to sustain and entertain him for the rest of his life. But he found another thing to please him. He enslaved a group of pixies and had them give him fanciful visions with their power to summon glamors. Having a very strong mind, he could have resisted the addictive powers of these visions for over a hundred years, by which time he probably would've died of old age.

But Zumaki was attacked by a Mak Fawa. This creature had probably arrived during the Phyrexian Invasion, and was already battered. But Zumaki was not content with simply destroying the attacker. He wanted to see if he could take it over with his mind. But the Mak Fawa had been an infiltrator, and although it was destroyed itself, the backlash made Zumaki's head explode.

From the remains of the two dragons a new dragon was created: with the body of the Mak Fawa and a twisted version of Zumaki's mind. It began to terrorize the countryside, with Vaan, the last survivor of the group of pixies, being forced to hide it's metallic body in a glamor, so it appeared as Zumaki. Vaan eventually lead a group of dragonslayers to the being's lair, but though they came close, they did not manage to completely destroy the beast.