Grimskull

You'll both love and hate this four-eyed bastard.

"Get off my ship, Space Marine!"

– Warboss Grimskull, when there was a Space Marine on his ship

Grimskull was the Ork Warboss leading the Ork WAAAGH! invading Graia and the secondary villain of the game Warhammer 40,000: Space Marine. He is also nigh impossible to kill; not even bloodletters can, and those things kill as a hobby.

Graia

Grimskull led his Waaagh! to Graia with the intent of looting the Forge World's Titans. Whilst in his cruiser, he encountered Captain Titus (who used a jump pack to board the vessel from above); whilst the Warboss tried to fill the Ultramarine's body with lead, Titus used the cruiser's cannon to destroy the bridge, making the cruiser crash land. Grimskull survived that wreck.

Grimskull encountered Titus again in Manufactorum Ajakis. While Titus tried to get the mysterious power source for Inquisitor Drogan, Grimskull (who was leading an attack in there), became interested in the device and sought to claim it as his. However, the power source became unstable and loosed a shockwave, which sent Titus plummeting to the sewers. Rather annoyed by this, Grimskull sent his Orks to retrieve it.

As Titus tried to activate the Psychic Scourge (a weapon apparently capable of destroying the Ork invaders), Grimskull arrived by dropship (a minelayer re-purposed for transport). As Grimskull was about to attack, he got blasted by psychic waves. However, to the Ultramarines surprise, this didn't actually kill the Orks, but instead opened Warp rifts that allowed the Forces of Chaos to enter the forge world. After getting back on his feet, Grimskull was then swarmed by bloodletters, which made him lose his footing and fall over the railings. Whilst Titus and the group were being incapacitated by the Chaos Lord Nemeroth's sorcery, Grimskull climbed back up, apparently defeating the bloodletters without so much as a scratch on him, and engaged Nemeroth in combat after saying the most badass thing an Ork has said in the history of 40k: "I ain't so easy ta kill." That said, he got his ass kicked after a short fight which ended with Nemeroth pushing him off the platform, but Grimskull grabbed on and yanked the Chaos Lord down with him.

Grimskull made his final appearance trying to kill Titus; however, the Ork was bested by the Ultramarine and finally executed, his face blown off by a plasma/bolt pistol shot (depending on which of the two Titus has equipped at the time). Without their boss' leadership, the Ork horde quickly fell into disarray, in the face of the arrival of an entire fleet of Imperial reinforcements.

Also worth a mention that when you go encounter him for the last time, he is there relatively unscathed and Nemeroth is nowhere to be found. Now, either the Warboss played dead or ran (highly unlikely), or Nemeroth is actually the one who lost and fled the fight after their fall, but it's more likely that the Chaos Lord simply wasn't interested in scrapping with some random Ork Warboss (what with there being probably millions of Warbosses in the galaxy, few being of any note whatsoever and virtually none of them having anything to do with Chaos or corruption) and went off to accomplish his goals instead.

Post-mortem, the events at Graia would be called the "Grimskull War" according to Warhammer 40,000: Boltgun. Even with Nemeroth hijacking his WAAAGH! over Graia with a Chaos invasion, the Imperium seems to at least see him as enough of a threat that the whole incident was named after him.

Trivia

• His coloring suggests that he's from the Evil Sunz. If they won, you know that "mysterious power source" would have ended up being the engine in his sick new buggy.
• Then again, his deep-seated desire for lootin' suggest a Deathskulls affiliation.
• He is one of the rare kind of Orks, since he actually has a bit of tactical awareness. This is shown when he orders his Orks to seize the planetary defense weapons and turn them on the Imperial ships, denying the Imperial ground troops any further support. Titus even remarked that he wasn't a fool, which is about on par for a Warboss.
• He is one of the very few Orks who can actually hit his target.
• He isn't finished with Titus.
• Titus is finished with him.
• He and the Orks depicted in Space Marine are apparently less grammatically challenged than the Orks usually depicted in other games (possibly a side effect of their leader being no fool).
• He's the only character to have a more traditional boss fight. Unlike Nemeroth, whose boss fight only consisted of really easy quick time events.
• His Mega Armour is powered by a totally badass V8, mounted on his back. It's also manly as all fuck because its only the arms and backpack. (It's also not Mega Armor; Grimskull uses his V8 Engine only for murder, leaving the rest of his body to tank plasma bolts. Take a look at him, he doesn't cover his green with metal, because that would take away from the Ork underneath.)
• His WAAAGH and him go out of their way to show Orks aren't all comedic relief like they are in other games, and the message you find is actually quite dark: We're talking multilated bodies of innocents filling bloodsoaked halls and workers wards, which Titus notes Orks do "for sport".
• He's fucking awesome.
• He shares his name with a Space Wolf from the Ragnar Blackmane books, but they've never met.
• And last but perhaps most importantly of all...He ain't so easy to kill

Quotes

SPACE MARINE! WOZ DAT S'POSED TA HURT? ARHARHARHARH!

Ork Gitz and Bosses
From da canon:	The Beast - Boss Snikrot - Boss Zagstruk - Ghazghkull Mag Uruk Thraka Grukk Face-Rippa - Mad Dok Grotsnik - Makari - Mek Boss Buzgob Mozgrod Skragbad - Nazdreg Ug Urdgrub - Old Zogwort - Orkimedes Tuska Daemon-Killa - Wazdakka Gutsmek - Zhadsnark da Rippa - Zodgrod Wortsnagga Kaptin Badrukk - Herman von Strab - Ufthak Blackhawk
From da gamez:	Brikkfist - Gorgutz 'Ead 'Unter - Kaptin Bluddflagg - Grimskull Mister Nailbrain - Orkamungus - Spookums - Stupid