"I ain't finished with you yet, Space Marine!"
- —Warboss Grimskull during the Ork invasion of Forge World Graia.
Grimskull was the Ork Warboss who led the massive Ork invasion of the Imperial Forge World of Graia by the million-Greenskin strong WAAAGH! Grimskull. The Orkish invasion met with great initial success and a force of Space Marines from the Ultramarines 2nd Company under the command of Captain Titus were sent to delay the Orks long enough to prevent them from seizing control of the strategically important Warlord-class Battle Titans built on the planet until a full Imperial liberation fleet arrived to secure the world.
Captain Titus and his Ultramarines first encountered Grimskull when they arrived at Graia and attempted to land on the world in a Thunderhawk gunship. Titus launched a Jump Pack assault upon Grimskull's Kill Kroozer and destroyed it by turning its own weapons upon it to eliminate its heavy guns which were cutting a swathe through the defending Imperial spacecraft. This action began a feud between the Warboss and the Space Marine Captain that Grimskull took very seriously. He next attacked the Astartes when Titus was attempting to retrieve the experimental power source that had been requested by Inquisitor Drogan, who had built a weapon known as the Psychic Scourge that he claimed could make use of the power of the Warp to interact with the Orks' own innate gestalt psychic field to destroy the WAAAGH! by literally ripping their mind apart. Drogan had placed the power source into the Manufactorum Ajakis' own power generator to maintain its defences from the Orks, but the Greenskins managed to find an alternate way into the manufactorum's depths. But when Grimskull saw the power source, which he called a "sparky thing," he felt he needed to possess it. Titus stood up to the Warboss' onslaught and managed to claim the power source first.
Titus next used Grimskull's continuing hunger for the experimental power source to lead him and his Orks across Graia to distract them from the Ultramarines' allies in the Imperial Guard, who were seeking to build a defensible perimeter that could last until the promised Imperial reinforcements arrived to retake the Forge World. This plan proved successful and the Ultramarines managed to get the power source to Drogan who inserted it within the Psychic Scourge and then activated the weapon. Unfortunately, the Inquisitor had lied to the Astartes and the Psychic Scourge was not actually a weapon intended to destroy the Orks, but an arcane device of dark, Chaos-corrupted technology capable of opening multiple gateways into the Warp. From one of the multiple gateways the device opened into the Immaterium all across the Forge World, sprang a great host of Bloodletter Lesser Daemons and the puppet Inquisitor's true master, the Chaos Sorcerer Nemeroth and his warband of Chaos Space Marines known as the Chosen of Nemeroth. Grimskull, who had chased Titus and the "sparky thing" to the Orbital Spire where the power source was placed into the Psychic Scourge, accepted these newly arrived foes with the usual laughing bravado of the Orks, but quickly was overwhelmed by a horde of daemons and fell from the height of the spire with many Bloodletters still clinging to him. The Space Marines were rendered helpless by Nemeroth's sorcerous powers and proceeded to have a brief discussion with the Chaos Sorcerer Lord, but Grimskull suddenly returned to the fight, proclaiming "I ain't that easy to kill!" After a short but intense grapple with one another, both combatants fell over another one of the Orbital Spire's railings and disappeared, leaving the Ultramarines free to dispose of the remaining daemons.
Unfortunately for Titus, the next time he encountered Grimskull, he would have to face the Warboss without the support of the two members of his company Command Squad who had accompanied him to Graia, Veteran Sergeant Sidonus and Battle-Brother Leandros. Grimskull fought the Ultramarines, using the Ork Boyz of his WAAAGH! as cannon fodder to distract the Ultramarine. Sadly for the Warboss, the tactic did not work. Grimskull's mighty armaments and followers could not stop his death at the hands of Titus. As he was laying on the ground, he said to Titus "I ain't done with you yet, Space Marine!" Titus cooly responded, "But I am done with you," and the Astartes executed the Warboss, ending his threat to the realm of the Emperor.
The Space Marine video game takes place in a parallel Warhammer 40,000 universe where the Ultramarines 2nd Company is commanded by Captain Titus rather than Cato Sicarius, making it unclear how Grimskull would transfer to the mainstream Warhammer 40,000 Universe.