Perjour's Corpse.
Perjour was a servant in the service of Zaros, first mentioned in the lore Twice Burnt. He was bound to a book that magically wrote everything he saw, heard and thought, which Zaros would read after every mission he sent Perjour to witness.
Zamorak casted a spell on Perjour and his book while they were watching the dancer Char perform. This spell altered the one Zaros had placed on Perjour, duplicating the book into Zamorak's possession, so Zamorak could read what was being reported back to Zaros. When Perjour was taken with Zamorak, Viggora, Thammaron and Enakhra, Zamorak altered the spell again, causing Zaros' version of events to show an altered version of events, one that hid certain parts from him allowing Zamorak to act without fear of Zaros finding out.
This altering of the spell was likely a help to Zamorak when he overthrew Zaros with the Staff of Armadyl (and presumably the Stone of Jas) as Zamorak's actions would not have been known to Zaros enabling him to plan unhindered.
Perjour died in the caves of the Elder Kiln, as you can read in his Journal when one does the quest The Elder Kiln. At the time of the Journal's writing Perjour mentions he is searching for the Kiln in attempt to raise an army for Zamorak. From reading the lore Twice Burnt and the Journal of Perjour, one can conclude that Perjour sided with Zamorak when he overthrew Zaros and served him up until his death in the tunnnel to the Elder Kiln.
His journal also mentions the fact that Char 'died' in a cave also. This is the cave explored in the quest The Firemaker's Curse although it is discovered by the player that Char is not, in fact, dead.
Trivia
- The name Perjour is very similar to perjure which is the willful act of swearing a false oath or affirmation to tell the truth, whether spoken or in writing.