You think you can walk out of here with that sparky thing? I'm Warboss Grimskull! this whole planet is mine!
~ Warboss Grimskull

Warboss Grimskull (simply known as Grimskull) is a powerful ork leader and one of the central antagonists of the Warhammer 40K video game known simply as "Space Marine".

Throughout the game, Grimskull hounds the Ultramarines, seeking to capture a mysterious weapon and believing (in typical ork-style) that he has the right to capture the weapon for himself and kill the Ultramarines for daring to try and "steal" what he wanted.

In fact, as Grimskull himself stated he believed the entire planet was his to loot and brutalize as he saw fit, it is very likely the true reason he wanted the weapon was that it gave him an excuse to fight the Ultramarines plus orks often grow in status via collecting "bits" (scraps of metal and the like, stolen from fallen foes and used as war-trophies and make-shift weapons and/or armor).

Ultimately Grimskull was slain in battle, his last words being "I'm not done with you yet, Space Marine!" - true to his nature as a relentless pursuer.

Following Grimskull's death the ork invasion is much less prominent as the battle focuses on stopping the Chaos Space Marines, who have managed to amass under the guidance of Nemeroth.