Even the accusation of being a Blood Mage, regardless of the truth of the matter, can be dangerous. To that end, the Collective is seeking someone to assist in such a matter.
To complete this quest, find and interact with the four different doors scattered throughout Denerim. The locations are as follows:Denerim Market District - 2 of them. Dirty Back Alley Dark Alley
To activate the doors on the PC version, you must move your cursor over the lower section of the door. Your cursor will turn a golden color, rather than grey.
Return to any representative of the Mages' Collective for a reward.