I planned to post detailed info about dragon rulers of the Underworld, but I'm really sick, so everything I can do is post their names with a brief description.
Grax Bloodfang - the Old Black Dragon - most cunning and evil of all dragons. He lives on the island located in the middle of the great underground lake;
Thrax Emberfire - the Red Beast - furious and insane monster, lord of the deepest parts of the Lava Caves;
Vrax / Morax - previously mentioned here, but I've changed my vision of the game since then, so I decided to dethrone him :D Lord of the Galroth Spire;
Tanax the Silver-winged - also known as the Great Traitor. Outcast hiding himself in the Megastructure; also it's possible that he is trapped there.
Forgive me the general shittyness of this post - as I mentioned before I don't feel good today.