Krushak, known as Sleeper by his human worshipers, was a powerful ancient Arch-Demon who served Beliar. He was summoned to the mortal world by five orc shamans, when they attempted to gain the power to defeat their enemies.
During the creation of The Barrier by the 13 Magicians, Krushak's presence in the Valley of Mines managed to warp the delicate structure of magic, and caused the barrier to grow out of control, engulfing the entire Valley and trapping everyone inside.
Xardas, through his studies of the area, and the Dark Arts managed to uncover the truth of the Demon that resided beneath the Valley, and that it was this powerful magical being that caused The Barrier to grow out of control. When The Player informs Xardas that The Brotherhood of the Sleeper were infact praying to an evil Arch-Demon, he ultimately decides that this demon was in fact, Krushak.
This is further confirmed by Urshak, who directly translates Krushak to "Sleeper" in human language.
Krushak bore the resemblance of a large Minecrawler. It walked on four legs, had two powerful arms ending in single claws, and a large head, which was protected with a helmet crafted in the symbol of Beliar.
It is unclear how physically powerful Krushak was, but it was clear that it was an extremely powerful magical creature, that could be sensed by other magical beings, and was the root cause of the Barrier growing out of control. One of it's main powers was the power of Possession.
Krushak managed to infiltrate weak and evil minds, and bend them to its will, it could be felt by magical beings such as the Magicians within the Valley, the Shaman of the Orcs, and the individuals who have enhanced their spiritual power while under the influence of Swampweed. Krushak used its connection to the weak minds of the Brotherhood while under the influence of swampweed to influence them, and ultimately bind some of them to its will.
Krushak could summon powerful demons, resurrect and reanimate the dead, and was worshiped and feared greatly by the Orcs.
Since it's summoning was due to the five Orc Shaman giving up their hearts in order to bind it to the living world, that was also its undoing. The Player manages to banish Krushak's spirit back to Beliar's Realm by piercing the 5 hearts with the 5 swords of the Shaman.
It appears that the power of the three gods can never be destroy, only transferred, and it is believed that after the banishment of The Sleeper, this power was transferred to the Undead Dragon. This is further evident since the servants of the Undead Dragon were the former Brotherhood of the Sleeper members of the Swamp Camp, changed into Seekers. Xardas also notes that his original plan was to take the power of the sleeper for himself, but was denied this opportunity when he was put under a spell shortly after arriving in Krushak's chamber. He would not be denied this power a second time, and ultimately takes it from the Undead Dragon after the events of Gothic II.