|“||Interesting company you keep, Magnus Chase. Someone is hiding, but I can smell him. Which one is he, eh? Perhaps I will get a feast today after all!||”|
The Fenris Wolf (pronounced "FEN-riss"; often spelled Fenrir and pronounced "FEN-reer") is the wolf son of Loki and the jötunness Angrboða, who is destined to start Ragnarök after Surtr severs his binds. He is the brother of Hel and Jörmungandr, as well as the half-brother of Sleipnir and Samirah al-Abbas.
Fenris is the one responsible for sending wolves to Magnus Chase's apartment and killing his mother Natalie (this is to affect Magnus and make him find Sumarbrander/Jack to cut the ropes that bind Fenris). He is also responsible for killing Blitzen's father Bili and for convincing Odin to make Sam a Valkyrie.
Magnus and his friends arrive on Lyngvi to tie a new rope around Fenris to stop him from escaping the island. Gunilla, her Valkyries, and Magnus's hallmates arrive on the island to arrest the group, but luckily the hallmates turn against Gunilla and take Magnus's side. Fenris takes a sniff at them and realizes that something delicious is hiding among them, but no one knew what he meant.
Whilst the others battle Surt and his fellow fire giants, Fenris tries to attack Magnus, but is stopped by Sam (in the form of a lion), who battles him. This gives Magnus time to attach the new rope to Jack, who then ties it around Fenris and defeats him.
Back at Hotel Valhalla, Magnus finds what Fenris Wolf meant with his smell. X, a half-troll, was actually Odin in disguise.
| Then there was the little one...
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Much like his father, Fenrir is manipulative, sly, humorous, charming and very ruthless. He likes to trick those who come to his prison, triying to look harmless and contained, he lures his victims with his charmspeak into getting closer, so he can attack and kill them. He did this to Blitzen's father. Fenrir is also very arrogant, due to the fact that he can't be killed before the Ragnarok, he claims to be invincible.
In The Sword of Summer, Fenris is described as larger than an average Labrador retriever, but definitely not much larger than a regular wolf which surprises Magnus. His legs are long and muscular, built for running, while his shaggy gray coat swirls with tufts of black. Fenris' eyes shine with a blue light that "sent a xylophone mallet down the back of [Magnus]' ribcage". According to Magnus, Fenris' eyes are colder and more intelligent than those of any predator he'd ever encountered – animal or human. In addition, the wolf's black lips curl in "a very human sneer". Despite Fenris' gleaming white fangs and his utter hatred for the wolf, even Magnus admits that he is a handsome animal.
Being one of the triggers of Ragnarök, and fated to devour Odin himself during said event, Fenris Wolf is one of the most powerfull and feared monsters in existence; the Norse gods feared his power so much, that they decided to chain the beast, so he could not wreak havoc across the Nine Worlds.
- Enhanced Strength: Fenris Wolf has demonstrated great strength.
- Enchanced Agility: He is still menacingly nimble though bound, which showed when he fought Sam (in the form of a lion).
- Enhanced Reflexes: Fenris is fast enough to confront Sam, Magnus, Blitz, and Hearth at once. And the most dreadful evidences are the remnants who fell to his victims even he's confined to Gleipnir.
- Enhanced Bite Force: The most powerful as well as the deadliest trait is his mouth through which he has the ability to bite off Tyr's hand and kill Odin.
- Charmspeaking: The wolf's voice is described as "deep and rich", flowing over Magnus like molasses, "dangerously smooth and sweet". As a result, he is finally able to understand the fear Valhalla's thanes had of "the silver tongue of Loki’s children". Fenris could even talk Odin into choosing Sam as a Valkyrie and lure the victims to slacken and into his attack range.
- Telepathy: Fenris can give his sons an attack command to get to Magnus and his mother, affecting Odin to designate Sam as Valkyrie in far distance.
- Invincibility: Much like other destiny protected beings who would only be destroyed in certain circumstances, Fenris could only be slayed by Vidar during Ragnarök due to the prophecies of Norns/Fates, which, therefore, is why they're so fearful.
- During Ragnarök, Fenris is destined to kill Odin, but will in turn be killed by Odin's son, Víðarr.
- His name Fenrir means “he who dwells in the marshes”.
- According to some legends, Fenris is the eldest of Loki's three children.
- Ragnarök will befall as soon as his two sun/moon-chasing sons, Sköll and Hati, whose names connote scorn and hate respectively, devour the Sun and the Moon.
- Sköll, Hati or Mánagarmr may just be his other names.