Danarius is a a minor antagonist in Dragon Age II, but the main antagonist of Fenris' storyline. He is a Tevinter magister and Fenris' former master.
Danarius is one of the powerful magisters of the Tevinter Imperium, with interests in experimentation with magic and lyrium. He treats his slaves brutally and resents any of his possessions which elude his grasp.
Hawke and Fenris assault Danarius' estate. The mansion is heavily guarded by demons, though it transpires that the magister has fled to Tevinter.
It is revealed that Hadriana, Danarius's apprentice, has travelled to Kirkwall with a contingent of slavers to try and recapture Fenris. She will be killed at the end of the quest after revealing the identity of Fenris's sister.
Aveline Vallen will inform Fenris that his sister has been spotted. Fenris will ask Hawke to accompany him to the Hanged Man, where Varania is staying. However, it emerges that Varania is a mage, and being trained as the apprentice of Danarius, whom she has led to her brother. Hawke can either allow Danarius to take Fenris, or aid his/her companion in killing the magister. In the latter option Fenris grabs the exhausted Danarius by the throat and uses his power to kill him saying "You are no longer my master!".
If Hawke allows Fenris to be taken, they will receive a letter from Danarius, which includes payment, an account of Fenris's status, and an offer to tour the Arcane Hall of Minrathous, should Hawke ever find him/herself in the city.