He is an ex-member of the Companions. He was asked to leave because his "unorthodox" methods were offensive to other members. It is unclear what these methods were or how they were offensive, however, given his current status, it seems possible he used underhanded, duplicitous techniques that would have offended the Companions' sense of honor. He joined The Circle and became a werewolf prior to his expulsion.
He initially disapproves of the Dragonborn, showing a rude attitude, but after several successful assassinations, he begins to respect and even envy the Dragonborn.
If the Dragonborn is also a werewolf, however, he will immediately know and react a bit more amiably. Regardless of whether they are a werewolf or not, Arnbjorn will use little nicknames such as "lambshank," "beef roast," "tidbit," or "hamhock." He says not to take offense from the nicknames.
When returning to the Sanctuary after killing Gaius Maro, there is Astrid and some other Dark Brotherhood members standing around Veezara, who is wounded. Astrid will tell the player to go chase after Cicero, who attacked her and the other Brotherhood members in a fit of rage over their dislike of the Night Mother. Astrid also tells the player to find her husband, who chased after Cicero to protect the other members. After traveling to the Dawnstar Sanctuary, Arnbjorn is found heavily bleeding outside the black door, unable to continue.
Arnbjorn is killed during the assault on the Dark Brotherhood Sanctuary, where he dies in werewolf form while holding off Penitus Oculatus agents. His death is scripted and he will still die even if the Penitus Oculatus are killed first.
- "Should have figured Astrid would send you."
- "Gee, thanks for the help."
- "What gave it away?"
- "Do me a favor and kill that little jester twice. Just to make sure."
- He wears a unique short sleeved variation of the shrouded armor. If the armor is stolen and equipped, regardless of the character, it will have long sleeves.
- He never wears any sort of footwear or gloves, possibly due to his Lycanthropy.
- He is the only character who will never give any useful advice when asked for it before a mission, apart from Cicero, who does not have that dialogue option.
- He can sometimes be seen in the forge part of the sanctuary attacking nothing because of movements made by two-handed attacks. He may be seen using a dual one-handed power strike, despite using a two-handed battleaxe.
- He, like many other members of the Dark Brotherhood, thinks Cicero is insane.
- When he dies at the hands of the Penitus Oculatus, Arnbjorn will have the body piece of the Shrouded Armor in his inventory.
- During The Cure for Madness, Cicero calls Arnbjorn a "hulking sheepdog," in derogatory reference to Arnbjorn being a werewolf.