Alone is a companion quest for Fenris in Act III of Dragon Age II. Fenris asks Hawke to go with him to meet his sister during the day at the Hanged Man.
Walkthrough
Talk to Fenris in his mansion at the beginning of Act III. He will tell you that he found his sister and to meet her at the Hanged Man. Take Fenris to the Hanged Man during the day. A scene will occur with his sister. When it becomes clear she is behaving strangely, it's revealed that she sold Fenris out to Danarius, who at that point enters the Hanged Man. If you allow Danarius to simply take Fenris, there will be no combat. Handing Fenris over to Danarius will result in 
Aveline: rivalry (+5), Merrill: rivalry (+10), Varric: rivalry (+10), Sebastian: rivalry (+10), and 
Anders: friendship (+5) if they are in your party. If you support Fenris, then you'll be attacked. There are three waves of enemies (Slavers, Demons, Corpses) and Danarius. The scene afterwards will give some insight into Fenris' past.
Result
If Hawke gives Fenris back to Danarius, a letter will appear at the Hawke Estate from Danarius. The letter states that they have returned to Tevinter and Fenris's memory has been wiped again, now returned to his "compliant self". Oddly, the letter mentions some reward, though after opening the letter, none is included.
If you side with Fenris, then Fenris will kill Danariuis himself and then confront Varania. She says she betrayed Fenris because Danarius promised to make her a magister. You then have the option to let Fenris kill her Fenris: friendship (+10)or convince him not to. If Varric is in the party and Hawke attempts to convince Fenris not to kill Varania, he tells Fenris that "this is the last thing you want to do" (in reference to his own experience with his brother, Bartrand). Allowing Fenris' sister to live results in Sebastian: friendship (+5), Varric: friendship (+10), and Fenris: rivalry (+10). If you allow Varania to live and question Fenris blaming magic, you will receive a whopping Fenris: rivalry (+25).
If Varania is allowed to live, she reveals that Fenris, in fact, did wish for his lyrium markings and had competed against others for them. When he had been chosen for his future position he was granted a boon, which he used to free his mother and sister from slavery. She then states that "freedom was no boon", and leaves after telling Fenris that she felt he received the better end of the bargain. Completing "Alone" will trigger the companion quest [Questioning Beliefs - Fenris].
Bugs
- (Confirmed 360/PS3) After you hand Fenris over to Danarius, go right back into the Hanged Man and you'll go back to the cut scene with Fenris again. You can keep doing this for 5 Sovereigns and friendship/rivalry points. Oddly enough, you do not get to fight Danarius if you decide to side with Fenris. Also, Fenris' mansion will disappear from the world map and will no longer be accessible, but he will still be available as a party member.
- It is important to not go to the Hanged Man without Fenris after accepting this quest, as it can break the quest and make meeting his sister impossible (PS3 and Xbox). If this does happen, the only way for the quest to work is to go to the Hanged Man through Lowtown.
Tips
- The cutscene starts as soon as you enter so the game cannot be saved before the fight begins - save in advance!
- Don't forget you can enter Hanged Man from Lowtown which allows you to save immediately prior to the cutscene start.