Dragon Age 2 - Mirror Image is a companion quest for Merrill (the elven mage that joined you back at the Sundermount quest for Flemeth). For this walkthrough, players will travel back to the Sundermount Elven camp, and explore a cave. The quest ends with a climatic battle against a Varterral, a spider-like magical creature created by the elves.

1. The quest will become available to you once you complete the Blackpowder Courtesy quest.
2. Speak with Merrill in her home at the elven alienage in Lowtown.
3. Accept her quest.
4. Travel to Sundermount, to the same elven camp you visited right after you got to Kirkwall.
5. Speak with the Keeper Marethari at the camp.
6. Follow the quest arrow to the cave. 
7. Kill the skeletons in front of the cave, and enter
8. Follow the quest arrow in the cave. You will have to fight some spiders.
9. Loot Radha's corpse at the quest location.
10. Continue on to the new quest location, and loot the amulet there as well.
11. Continue on into the cave, killing spiders, and looting the last corpse.
12. After the cutscene, you will fight a mini-boss, a Varterral. The fight is not very difficult; just make sure to kill the spiders that spawn throughout the encounter.
13. Exit the area. You will have to kill some skeletons.
14. Return to the elven camp by following the quest arrow and speak with the Keeper.