Fenris Wolf

The Fenris Wolf (pronounced "FEN-riss"; often spelled Fenrir and pronounced "FEN-reer") is the wolf son of Loki and a giantess, who is destined to start Ragnarök after Surt severs his binds. He is the brother of Hel and Jormungand, as well as the half-brother of Sleipnir and Samirah al-Abbas.

The Sword of Summer
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.

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.

Appearance
In The Sword of Summer, Fenris is described as larger than an average Labrador retriever, but definitely not much larger than a regular wolf. 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.

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".

Trivia

 * Fenris is destined to kill Odin, but is also destined to get killed himself by Vidar, the son of Odin shortly thereafter during Ragnarök.
 * 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 moon.
 * Sköll, Hati or Mánagarmr may just be his other names.