Fenrus the Devourer

Arugal's tower
Follow the upper passages through Fenrus's chamber, Nandos's kennel, and Arugal's room. Collect The Book of Ur before leaving Fenrus's shelves behind, and finish any Arugal's Voidwalkers before advancing. Clear the worgs around Nandos before pulling him. Clear the worgen in Arugal's room before starting the teleporting fight.
Fenrus the Devourer
Elite BeastFenrus tests poison removal because Toxic Saliva drains mana as well as health during the melee fight.
Abilities

Toxic Saliva
Inflicts 17 Nature damage to an enemy every 6 sec. Also drains 20 mana every 6 sec., transferring it to the caster. Lasts 2 minutes.
Remove Toxic Saliva as a poison. Leaving it active costs mana as well as health, even after Fenrus is defeated.
Tank- Keep Fenrus's attacks on you so the healer does not have to take his melee damage while also managing Toxic Saliva.
Healer- Remove Toxic Saliva promptly, particularly from a mana user; healing its damage alone leaves the mana drain running into the next pull.
Damage- Remove Toxic Saliva if your class can cure poison. After Fenrus falls, finish any Arugal's Voidwalkers before advancing, focusing one so Dark Offering cannot sustain the pack.