The Abandoned Slaver Den is a former holding area for slaves hidden on the coast just outside Kirkwall. At the Tevinter Imperium's height, there were many such places throughout the Free Marches and attacks on them from rival slavers were common.
This section contains spoilers for: Dragon Age II.
In Act 2, Tevinter hunters catch up with Fenris. After defeating them, Hawke enters the den to kill Hadriana, the magister apprenticed to Danarius. While in the Slaver Den, Hawke encounters one of Hadriana's slaves, an elf woman named Orana. Hawke can chose to abandon her to her fate or help her.