Before the Great Cataclysm, in which the Deviant race capital would be lost, Crom was called upon by the people to rid of the demons of the lands. And those he answer one the few prayers, giving the people of the land three ironbound books. Once the Priest read from this tomes flames destroy the minor demons and imprison the all-mighty Shuma-Gorath under a mountain in the Thurian continent, which would later become Cimmeria. By the time of the Hyborian age, Crom had vastly increase his powers, do to common worshipers upon the land and warriors who chanted his name before battle, in this time he held the power of primordial god.
One key different between the upcoming immortals and him, was that Crom didn’t value humanity. He was grim and rather egocentric, never interfering with humanities affairs.
The sorcerers Kulan Gath and Vammatar both tried to free Shuma-Gorath as the demon could great empower both of them. There plan was to use the ironbound books which they had stolen and claim to the top of Crom’s mountain. But before such thing could star the grim god saw this and strike them down with terrible thunder that split the land. While both of them were unharmed they were separated leading to them to believe that each other was dead. Those the plan was abandon.