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, 9th edition
Mjolnir Foehammer to me!"*cough* The Anvil of Fenris is your Wolf Guard Battle Leader in Terminator Armour. His standard Lt. aura is mostly a secondary role; his primary one is flattening enemy Characters and Monsters. S5 and A4 puts him on top of other WGBL's, in conjunction with his buffed thunder hammer which he can YEET at an enemy unit. R12", assault 1, S10, AP-3, D3, gains +1D against Character or Monsters, with the melee profile being that same S, AP, D, and rules, but also does NOT inflict a -1 hit penalty. His Champion of the Kingsguard rule lets him re-roll failed hit rolls in melee against Characters as well. WS2+, re-rolls hit rolls against Characters, and gains +1 to hit rolls from his Chapter Tactics mean you should really always be making four hits against Characters. He's also the only Marine so far to have kept an 8th edition storm shield, meaning he also has a 3++ invulnerable save.
- He's 120pts compared to a Battle Leader Hammernator's 105pts. For those 15pts, you gain a powerful weapon that outshines most relics, +1S, +1A, a 3++ invulnerable save, and an excellent character smasher. The only reason you'd take a generic Battle Leader is for relics or a Warlord Trait of your choice instead of Arjacs fixed Warlord Trait.
- An absolute auto-include if you're running an army with Terminators.