"I ain't finished with you yet, Space Marine!"

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.

Now, in the ending you're used to, this all went off relatively well. But since I don't like things going well in 40k, this is an alternate timeline where Grimskull absolutely BITCHED Titus and killed him, and the Orks successfully took over Graia, now as for what you're gonna be... I have no clue, maybe you're the last guardsman on the planet, or maybe you're part of a liberation force, go off, I don't care.