Thaerisk Cemphier is the current leader of an underground organisation of druids called the Crux Eqal and the author of the Book of the Underworld. He is heavily involved in the While Guthix Sleepsquest. He can be found initially upstairs in the house with the 'crystal key chest', just west of Pikkupstix in Taverley. After the player talks to him for a while, two Assassins will appear and attempt to kill him and the player. He uses earth spells when defending himself. When the player kills them, Thaerisk requests the player to go to Varrock to continue the quest and offers to sell them a Snapdragon seed later in the quest. Afterwards, he moves to the White Knight's Castle and remains there upon completion of the quest.
He also appears in Ritual of the Mahjarrat quest, the sequel of While Guthix Sleeps. Near the end of the quest, Sir Tiffy Cashien wants to hide the Stone of Jas again. To teleport away the Stone, he summons Thaerisk. Together, they manage to teleport the Stone of Jas to an unknown location, which it is later retrieved from by Sliske. After the quest, he returns to Taverley.
He says he has lived in Taverley most of his life and was initiated as a druid ten years before the events of While Guthix Sleeps. He claims to have travelled around most of Gielinor and seen tragedy and heartache, triumph and beauty. He also feels that many of his fellow druids are a bit too absorbed in local matters and should look at the larger picture, and thus organised Crux Eqal. He says he feels angry that while weaker people suffer, stronger people oppress those weaker people.
"Thaerisk Cemphier", when pronounced properly, sounds like "They risk some fear." This possibly hints at his reputation, but more likely because most things that have to do something with Temple Knights have name plays, as the Crux Eqal works together with the Temple Knights.
Thaerisk may die during The World Wakes if he fights Enakhra or Zemouregal and will be replaced by a druidess who's also a member of the Crux Eqal.