Justice is an Act 3 companion quest for Anders in Dragon Age II.
Acquisition
This quest becomes available after completing the Check on Anders and leaving Darktown for any other location. It is not available if Hawke told Anders to go away after Dissent in Act 2.
Walkthrough
Travel to Ander's Clinic in Darktown and speak with Anders. He will inform Hawke that he has found a recipe for a Tevinter potion that will free Justice from him without harming either of them, but he needs Hawke's help to collect the ingredients. If you bring Sebastian along, he will make a funny comment. Also, if you select the aggressive option in the very end of the conversation ("Is that.. all?"), Anders will unexpectedly crack a joke in reply. If you refuse to help, the quest is completed.
The first of the two ingredients, Sela Petrae, is found in the sewers of Darktown. Head there with or without Anders (he will appear at the entrance and you will have the option to choose remaining party members). If you are romancing either Fenris or Merrill at this point Hawke will engage in a brief conversation with Anders who will suggest that Hawke be with someone more open-minded. You will have to collect five deposits of Sela Petrae and along the way you'll encounter Lyrium Smugglers, Carta Thugs and some Apostate Mages. You will also be able to find a part of The Enigma of Kirkwall near the foot of some stairs in the southern section.
To continue the quest, head to the Drakestone Mines in the Bone Pit outside Kirkwall with or without Anders (again, he will appear at the entrance and you will have the option to choose remaining party members). Fight your way through Poisonous Spiders, Corrupted Spiders, and a Monstrous Spider to collect five deposits of the second ingredient, Drakestone. Don't forget to pick up the deep mushroom in the mine.
Head back to Anders Clinic and speak with him once more. He will reveal that the potion was a ruse to get Hawke's help and ask for one final favor: to distract the Grand Cleric in the Kirkwall Chantry for an undisclosed reason. No matter what you say, Anders will not reveal his plan. Go to the Chantry and speak with Grand Cleric Elthina about mages. If Sebastian is with you, he will leave as soon as you enter the Chantry. You will not be able to choose every dialogue option before Anders appears by your side claiming to have been looking for Hawke everywhere.
If you are not Anders' 100% friend, talk to him in the Anders' Clinic or in the Hawke Estate (if he moved in) to finish the quest. He will tell that the spirit inside him is exultant. He will refuse to explain what happened in the Chantry, but will say that the war will happen: the clock is ticking down and it will be midnight soon. If you are Anders' 100% friend, this conversation will not trigger, the quest will end automatically and another conversation during the follow-up quest Questioning Beliefs will trigger instead.
Notes
- After Hawke refuses to gather ingredients or to distract Elthina, Hawke gains a special option in the dialogue with Elthina regardless of Anders' friendship/rivalry score. This option is available only before you accept the quest Faith. Being warned that the apostates are plotting against the Chantry, Elthina replies that mages have threatened her before and she will not succumb to fear. If Anders is in the group, Elthina mentions him.
- If Hawke helps him with both his requests, then:
- Rivalry Hawke gains a special option in the dialogue with Cullen. Being warned that Hawke's associate is plotting against the Chantry, Cullen replies either that he doesn't need to ask which known apostate Hawke is talking about and will pass on the warning (if Anders is in the group) or that he has already warned both Meredith and Elthina about the rebel Anders, and a force will be sent to apprehend him soon (if Anders is not in the group).
- Friendship Hawke can't warn anybody.
Warning Elthina or Cullen results in 
Anders: rivalry (+15), 
Sebastian: friendship (+15), if they are in the group. Nothing in the plot actually changes as a result of any of these conversations.
Companion Reactions
Accepting the quest via 'Kind' / diplomatic responses:
- Anders: friendship (+15)
Fenris: rivalry (+10) (No change with maxed friendship, and Questioning Beliefs has been completed.)
Accepting the quest with "You're wrong" followed by all Humourous responses, or with "What does that mean?" followed by all Kind/diplomatic responses gives Anders: friendship (+10). Accepting quest with "What does that mean?" followed by all Humorous responses Anders: friendship (+5).
Accepting the quest with "You're right" followed by all Aggressive reponses gives Anders: rivalry (+10). Accepting the quest with "What does that mean?" followed by all Aggressive reponses gives Anders: rivalry (+5). (Don't reject request.)
Agreeing to help Anders get into the Chantry:
- Fenris: rivalry (+5) (No change with maxed friendship, and Questioning Beliefs has been completed.)
Refusing to help Anders getting into the Chantry unseen can net you a massive
- Anders: rivalry (+30) (in romance with Anders, no rivalry points after exhausting all other options in conversation)
- Sebastian: friendship (+15)
- Fenris: friendship (+5)
Agreeing to help Anders get into the Chantry at the end of the quest automatically nets
- Anders: friendship (+10)
Result
You will not witness the outcome of Anders' intent until the main quest, The Last Straw, where his final plan will be revealed.
- Doing this quest is not required for Anders to end up going the final route in the storyline. You can leave this quest just sitting in your log and The Last Straw will still pop as normal.
- If you helped Anders, his Renegade's Coat will recolor black after talking to Anders either as a part of the Justice quest (if you are not his 100% friend) or during the follow-up quest Questioning Beliefs (if you are his 100% friend).
- If you refused to help Anders from the start or refused to distract the Grand Cleric later, you can not subsequently talk to Anders and the armor stays the same.
Rewards
- 800 XP(on grabbing the fifth Sela Petrae in the Darktown sewers)
- A The Enigma of Kirkwall can be found in the Darktown Sewers
- 800 XP(on grabbing the fifth Drakestone in the Drakestone mines)
- Deep Mushroom in the Drakestone Mines, on the north side of the first room down the stairs from the entrance
- Also keep your eyes peeled for a Warden's Oath amulet and a Griffon-Feather Belt in the Mines
- 1200 XP(quest completion - if you help Anders distract Grand Cleric Elthina at the Kirkwall Chantry - also completing the follow-up quest Questioning Beliefs when you talk to Anders at Ander's Clinic at the same time)
Bugs
- The icon for Hightown will have the quest marker over it, while the actual quest takes place in the Chantry.
- The quest can glitch and show up as still active in journal. (Fixed in 1.03)-- Can still happen in 1.03
Trivia
- The items that you collect are two of the components for gunpowder, the secret of which is supposedly only possessed by the Qunari in Thedas (cf. Blackpowder Promise, Blackpowder Courtesy). Sela Petrae is saltpetre (Latin: sal petræ), which can be extracted from urine, guano, and the like. Drakestone is sulfur, which can be found in many mineral deposits. (The third component, charcoal, is probably plentiful enough that he doesn't need assistance for that ingredient.)
- When Anders finds Hawke speaking to Grand High Cleric Elthina she greets him with a sly pun about hoping he "...found a balm for his troubled heart here." With her accent, balm sounds much like "bomb." This is foreshadowing the plot and/or a possible reference to a similar pun in Monty Python's Life of Brian.