The Moonshae firbolgs believe that Grond carved the Moonshae Isles out of a single piece of rock and that the leftovers became the firbolg, while the rubble became the dwarves. They claim that the dwarves slipped him up and he fell into a crevasse that he'd cut with his axe and then a sheared-off mountaintop fell on him, trapping him there.
A less apocryphal theory is that his mother Hiatea granted him dominion over the Moonshae firbolgs to teach him responsibility while she concentrated on other matters. It is unknown how he came to be trapped under a glacier on Oman's Isle but being incarcerated so left him to drift into a deep slumber.
His supplicants devolved into a barbaric race, used as pawns by the powers who attempted to wrest control of the islands away from the Earthmother. They remained as such until the Era of Upheaval when he woke up and made a deal with both Helm and Talos to have their followers free him in order to gain a strong foothold on the Moonshaes. When those followers opened a way out of his prison with the Silver-Hafted Axe, Grond broke his accord with both gods and sided with the Earthmother against them.
Grond aimed to encourage his followers to abandon their degenerate ways and aspire to be more like the mainland firbolgs, including becoming reclusive from humans and demihumans. To achieve this, he sponsored a migration of his followers to Oman's Isle and converted tribal shamans to the ways of true priests. He hoped to have achieved his goal within a century of his freedom but it seems he didn't anticipate the encroachment of Faerie on the archipelago as Oman's Isle was dominated by formorians and their cyclops servants by 1479 DR.