Wayward Son

Head to the Alienage to talk with Arianni and Thrask. Arianni's soon is a half-elf mage that seems to have disappeared. You can follow Thrask back to the Gallows to learn more about the situation. When ready, travel to Lowtown at night to meet with the disgraced former Templar Samson.

With your new info gained from Samson, head to the Docks and into Arthuris' domain. Disarm whatever traps are around with your Rogue and take down the cannon fodder enemies. You'll enter a smaller corridor where you'll have to deal with both the pirate slavers and abominations on the loose. Area effect magic works the best here, as does luring out enemies and taking them down one at a time. The abomination and the shades it summons are the biggest threat, so you might want to keep a few human enemies around to cause a three way battle. After clearing the area be sure to loot the letter for Thrask, which can be used for a future reward.

With your new knowledge, head to Darktown and find Danzig and his gang. They can be easily killed with area of effect magic and a few debuffs. However, a second wave of enemies will arrive with some heavy firepower and good defenses. It's a good idea to retreat back up the stairs and fight each enemy one by one. When everyone is dead loot Danzig's body for the slaver's location.

Travel outside of Kirkwall and to the Wounded Coast. Fight anything you encounter on the way and enter the Slaver's Cavern. Kill more Slavers and you'll eventually find the Slaver leader Varian. You can pay him for Feynriel's freedom, or fight off the Slaver goons. If you want to retain some money, go ahead and kill the enemies inside. After either outcome, return to the Gallows to blackmail Thrask with the letter.