I recently re-read the Discworld novel Pyramids and I noticed something I missed last time. The high priest Dios mentions an island a few times and how he has to do something there because his body aches. Is it ever described just what he is doing there or is it only hinted at?
This "island" you're referring to is in fact the Necropolis, literally a city of the dead with streets of small tombs and mausoleums surrounding the enormous pyramids. The city of Djel is on one side of the river with the (far larger) Necropolis on the other side.
Dios regularly travels into the necropolis to take advantage of the time-reversing effects of the pyramids in order to attain a form of immortality:
He longed for the necropolis, and the cool silence among his old friends, and a quick sleep after which he’d be able to think so much more clearly…
Whenever he begins to feel old, he sleeps in the pyramid that had been constructed for him by the first Pharaoh and then returns refreshed and several years younger:
(note, edited for brevity)
At last they came, like a tide, to yet another pyramid. It was small, low, dark, half-concealed in drifted sands, and the blocks were hardly even masonry; they were no more than roughly squared boulders. It had clearly been built long before the Kingdom got the hang of pyramids. It was barely more than a pile. Hacked into the doorseal, angular and deep, were the hieroglyphs of the Ur Kingdom: KHUFT HAD ME MADE. THE FIRST.
It was a small pyramid, mazeless, without traps, just a stone passage leading upward. Tremulously, expecting at any moment to see unnamed terrors leap out at them, the embalmers followed the king into a small, square chamber that smelled of sand. The roof was black with soot. There was no sarcophagus within, no mummy case, no terror named or nameless. The center of the floor was occupied by a raised block, with a blanket and a pillow on it.
“Look at the sodding torch, Gern!” he shouted. They looked. Without any fuss, turning its black ashes into dry straw, the torch was burning backward.
At the end, we see him thrust back in time to the first days of the Empire, suffering from memory loss. We're left to conclude that he is trapped in a timeloop with no means of escape.
I understood that he went to the island to rest under his own pyramid, which explain why he did not age.