The Illearth Stone is a bane of terrible evil lurking in the depths of Mount Thunder. It is described as "one of" several such objects buried deep in the Earth, presumably by Lord Foul when the Creator was constructing the world. Drool Rockworm had found it and was learning to use it prior to the events of The First Chronicles of Thomas Covenant.
The power of the Illearth Stone was immense. It was capable of altering weather, mutating animals, and corrupting Earthpower to a profound degree. It seemed to operate in favor of disrupting the natural order, warping the Laws that regulated the existence of the Land, though it is possible that its full power was never unleashed.
Use by an unskilled mortal being such as a Cavewight apparently sickened the user, accelerating aging and illness. It is likely that even if the mission to Mount Thunder had not ended in his death, Drool Rockworm would have died soon afterward regardless. (Contrast his appearance at the beginning and end of Lord Foul's Bane.)
After the death of Drool, the Stone fell into Lord Foul's possession, as he originally intended; even though his enemies regained the Staff of Law, which was also in Drool's temporary possession, he was well content with his prize. With its power, Foul was able to corrupt Bloodguard to his service and awaken the Lurker of Sarangrave Flat.
Foul shaved three splinters off the Stone and gave one to each of his Ravers. It was the power of these fragments that allowed the Ravers to possess three Giant triplets, assuming the identities of Fleshharrower, Kinslaughterer and Satansfist. They used the power to assault the friends of the Land, causing terrible damage, including the massacre of the Giants at Coercri, the raising of two monstrous armies, and the siege of Revelstone.