A long-dead powerful wizard and a previous head of the Venerable Council of Seers. He appeared in The Light Fantastic and took part in the Rite of AshkEnte to summon Death so that he could explain why the Octavo had activated and cast a change spell encompassing the disc.
When the eight orders of wizardry competed to find and locate the last spell of the Octavo, and the wizard Rincewind who was carrying it, all the heads used different methods; magic mirrors, demons, runes and scrying. Treehallet claimed that he 'has questioned the very bones of the country, yea, and the deep stones and the mountains thereof.' They did not appear to have provided him with an answer.
Treehallet was killed, along with the other order heads, by the wizard Trymon who was under the control of things from the dungeon dimension. He was turned to stone, and his statue later moved into the University grounds.