This quest is obtainable if you've either imported a save from Dragon Age: Origins in which Prince Bhelen was awarded the throne of Orzammar, or you load the pre-made history Martyr, which has the same condition. Lord Renvil Harrowmont can be found in the Docks district of Kirkwall during the daytime.
As you come upon Lord Renvil Harrowmont and his guards, they are attacked by several Carta Thugs and a Carta Assassin. Once you've defeated the would-be assassins, Harrowmont asks you to help him clear the district of Carta members trying to assassinate him. From here you have the option of helping him out (which nets you a couple of gold in the mail), or turning on him when you meet the first set of Carta criminals.
If you decide to assist Harrowmont, head towards the westernmost end of the Docks and eliminate the three groups of Carta along the way. The first group leader gives you an offer to kill Harrowmont, which you can refuse or accept. If you refuse, proceed to kill them. Be wary of this Assassin and use the full strength of the party to defeat him before he does too much damage. If anyone is KO'ed but you survive the fight, simply blow the party summoning horn near the Qunari Compound to heal your injuries without using kits. Withdraw back through the east if you need room to stretch their forces thin. The doorway at the very eastern end provides a very tight space that makes it difficult for the Assassins to actually get behind you to backstab (Woodrow's Warehouse during Finders Keepers, for reference). Carta enemies are vulnerable to cold damage, suffering double the default value, making an Elemental mage and/or anyone wielding weapons enchanted with Rune of Frost a great choice. Beyond this, proceed carefully to challenge each group and do not advance too far west mid-combat, lest you provoke more enemies than you can handle. The final group has another Assassin who shows up after a few kills, so conserve your strength until he arrives. Every other enemy is either Critter or Normal rank and falls easily enough.
If you decide to assist the Carta in killing Harrowmont, head back to where you spoke with Harrowmont and he and his guards will engage you on sight. They are simply Elite-ranked sword-and-shield soldiers, none of the perks of "Commanders," just extra health. Unlike the Carta, these dwarves have the same resistances as Mercenaries, i.e. resist cold and weak to Nature (and appear to be the only dwarves in the game with these resistances), so take these into consideration.
The quest concludes after you receive your payment from whomever you sided with.
If you decided to help the Carta eliminate Harrowmont, then the entirety of house Harrowmont dies with him.
- 50 XP (initial encounter)
- 400 XP (quest completion)
Assisting The Carta:
- 3 (quest completion)
- 400 XP for killing three Carta groups (100 + 100 + 200)
- 1 (quest completion)
Additional rewards in Act 2:
- 5 (from a letter):
Requires: 22 strength
Immunity to stun (from the same letter)