The Argonian Mission is a safe haven for escaped slaves. It can be found in the southwest corner of Ebonheart on the largest island. It is adjacent to the Skyrim Mission, which is a haven for the Nordic slaves of Morrowind.
Within the mission one can find members of the Twin Lamps faction, who are opposed to slavery. These members (and others throughout Vvardenfell) will give the player quests that concern slaves. Once the player has freed enough slaves, they may receive a corresponding reward for their efforts.