Dissent is the companion quest for the Grey Warden mage, Anders, who joins your party in Act 1 during the Tranquility quest. Players will battle a secret group of Templars trying to inact the "Trainquit Solution", a plan to make all mages Tranquil (ie, take away their magic power and effectively lobotomise them in the process). After you finish the quest, you can also quickly do the follow-up Questioning Beliefs quest.

1. To receive the quest, speak with Anders in his clinic in Darktown (the same place you first met him).
2. Accept the quest.
3. Travel to the given quest location in Darktown to enter the Gallows Dungeon.
4. Travel through the dungeon. You will fight some dwarf mercenaries along with some archers.
5. After the cutscene, start fighting the templars.
6. Focus down the main templar quickly, and then dispatch his friends.
7. Note: Don't forget to loot the main templar, as the note on his body is needed for the end of the quest.
8. Head out of the area after the cutscene.
9. Decide what advise you have for the mage girl you saved.
10. Speak with Anders in his clinic to finish the quest. 
11. Note: you can now also complete the Questioning Beliefs quest for Anders as well.