GW decided that having Odin, Loki, Tyr, and Heimdall/Bragi(?) wasn't enough, so they decided to make a Thor, complete with magic throwing hammer, to round out their pantheon a bit. Arjac Rockfist (likely pronounced ar-yak) is currently one of the toughest Wolf Guard members in the Space Wolves Chapter. A man of few words and with a lot of brains and brawn, though he always says that he's unfit as a leader and would rather remain a warrior first and foremost. Once a smith and member of the Bear Tribe of Fenris, Arjac was already known as a giant among his people. When the Space Wolves found him, he joined the Iron Priests, for he really enjoyed working in the forge. That is after a certain event that changed his career...
One particularly hot season on the Iron Islands on Fenris caused thousand of Kraken Spawns to come out of the seas. During that time the Iron Priests were too absorbed in making awesome wargear that their senses were dulled. When they realized what was going on, the senior Iron Priest Hengis Blackhand locked up everyone in giant vaults so that the Kraken Spawns wouldn't get to them. Arjac on the other hand thought of it as SHAMEFUR DISPRAY and smashed through the vault gates, freeing everyone and readying himself to fight the chitinous abominations. Before he readied himself, a dozen of volunteers joined him in order to aid Arjac.
When Space Wolf reinforcements finally arrived, the sons of Russ noticed a single figure hammering off the waves of Kraken Spawn. When they finally reached Arjac, the battle was over, the forge was saved and every last of the spawns dead... with Arjac buried under the corpses. For this act of unquestionable badassery and valour, Arjac was promoted to the Wolf Guard by Old Wolf Logan. Nay, he became the personal champion of the Great Wolf. Even before becoming a Space Marine. That's how badass he is!
Arjac, apart of his suit of Terminator Armour, is armed with a unique Thunder Hammer named Foehammer that he can throw at an enemy and teleport back into his hand thanks to a build in teleporter (making it the 40K equivalent of Mjolnir), and a Storm Shield named the Anvil Shield that allows him to bash his enemies as hard as he can with his hammer.
Even to this day, the Wolf Guard still misses chilling out in the forge along his Iron Priest buddies, but every other Space Wolf will think that he's better off as a Wolf Guard due to the Titan sized awesome he generates.
On the Tabletop
Arjac Rockfist is a BEAST. This fucker has well earned his status as The High King's champion. As far as rules go he has the typical Wolf's ATSKNF and Acute Senses plus counterattack, stubborn, and independent character. His Anvil of Fenris ability means that if he isn't running alone he can only run with Terminators or Wolf Guard (Terminators), and since he's the Chapter Champion accepting and throwing out challenges is mandatory which can be problematic, but more on that later. As far as gear goes he has Terminator armor and two relics, his Anvil Shield and Mjolnir. His anvil shield gives him Hammer of Wrath and Eternal Warrior, and his hammer is StrX2 concussive and unwieldy, but it's also 6" ranged assault 1 concussive, as he can throw his hammer and it comes back. Arjac is T4 2+ 3++ eternal warrior with strength 10, and he plays like a poor man's Darnath Lysander. He'll destroy MC's, but he doesn't have the attack volume to get out of tarpits without help. He's good in a challenge, but not unbeatable: he has 3++ and EW, yes, but he's striking at Initiative 1 and he only has 2 wounds. If you put him against a character or squad with a fair amount of AP2 attacks or lots of rending, he'll probably die before he can even strike. Very powerful but situational.
Arjac Rockfist is a BEAST, still. He has good stats and a 2+ save and 3++ from his storm shield and in addition his shield reduces the amount of damage of incoming attacks by 1. He can throw his hammer up to 12", at S10, so that's nice as well. Also, his special hammer does not suffer the -1 to hit normally associated with thunder hammers. His special rules give wolf guard an extra attack within 6" and re-roll wound rolls of 1 for space wolves within 6". Since he's a wolf guard (and space wolf) himself, this nudges him up to 5 attacks. Against characters is where he really shines, gaining re-roll failed hit rolls and his damage goes from D3 up to flat 3 against both characters and monsters. And at S10 he's usually wounding characters on a 2+ anyway. So 5 attacks, hit on 2+ with re-roll, wound on 2+ with re-roll, AP-3, 3 damage? Sign me up.
His downside is that he's only movement 5, which is mitigated somewhat by being able to teleport onto the battlefield.
If you want to play wolf guard terminators, this is your go-to-guy. Add a Wolf Lord or Logan for extra fun. It won't make wolf guard terminators competetive, but it helps them out a lot.
Note for skirt-wearing emo-marines: If you want to use 'the lion and the wolf' stratagem, get Arjac. With +1 WS, +1 S and +1 Attack, this dude is unaffected by -1 to hit penalties, and gets 6 S12 attacks. He will put a serious dent in even Guilliman.