Pythius the rotten is an undead dragon living on Beregar caves. He claims to have obtained his treasure from the civilization of Yalahar when they started their downfall. You may challenge him as a part of The Hidden City of Beregar Quest if you have completed the The Queen of the Banshees Quest. (Or atleast received the kiss from the banshee queen). If you accept his challenge you can then access the teleporter next to him that leads to the dragon lord lair if you give him something for his treasure.