The Obsidian Tribunal is a faction mentioned during Sliske's Endgame if you raced against Saradomin and he won. Virtually nothing is known about the Tribunal, save that they are not a god, and they control a host of worlds, on one of which they held Adrasteia, Saradomin's daughter. Nex discovered her there and rescued her, holding her within the Frozen Prison, so Zaros could use her as a bargaining chip if Saradomin won the Stone.