Just in case there are still people playing the “Leliana’s song” DLC for Dragon Age: Origins wanting to earn the “Vendetta” achievement, here’s a brief list of what to do. As I found most of the search results on Google rather useless, here’s the definite info:
Vendetta Achievement

  • Steal the goods from all three merchants’ storage boxes. Do, however, NOT rotate the stolen goods among the merchants, but put them on the drunk captain of the guard’s body.
  • Take the obscene piece of underwear from the noble you had to embarrass and leave it on the captain.
  • After killing the bigmouth, loot his corpse and drop it near the captain.
  • When following the mages collective’s trail and finally finding the hidden storage (a pile of rubble close to the gate leading to the alienage), you’ll be attacked by a mage and a golem. The mage will drop a ring that can either bve equipped by your group’s mage, sold or – you guessed it – left on the unconscious guard captain.

After signalling Marjorlane that you’re done, she’ll praise your work and you will recieve the Vendetta achievement.