Grimnir was born as one of the first three dwarfs alongside Grungni and Valaya. Before the coming of Chaos, he was responsible for defending the fledgeling dwarf race from monsters, the most famous instance of which being his battle against the first dragon which was said to have created Black Fire Pass.
During the great Chaos incursion, Grimnir went on a quest to the Chaos Wastes, seeking the portal to the Realm of Chaos. Knowing that he would never return, he tatooed himself and shaved his hair into a mohawk, becoming the first Slayer. His last act before entering the portal was to entrust his axe to his son Morgrim. What happened next, no Dwarf can say, save that he faced a mighty doom.
Age of Sigmar
He returned in Age of Sigmar alongside Grungni, being found chained to a mountain. Valaya remains dead, making the marriage presumably awkward.
The Fyreslayers hold a belief that if enough Ur-Gold is gathered, Grimnir will be resurrected in time for the final battle they call Doomgron. They believe that he will hold a final judgement over the souls of the Fyreslayers, where he will resurrect the worthy and damn the unworthy.
Also of note is that several Fyreslayers are under the belief that Gotrek Gurnisson is the prophesized reincarnation of Grimnir. Gotrek himself hates being seen as such, mainly because he now despises Grimnir and thinks of him as a liar. Most of the time he just makes any Fyreslayer interested in following him swear an oath to leave him alone.