Breaker of Chains

You must be on the Apexis Crystal daily Scouting Missive: Stonefury Cliffs to access the area. The achievement is listed under Bloodmaul Stronghold but the zone is called Stonefury Cliffs.

Click on cages to free the prisoners inside — only caged prisoners count toward the achievement. Approximately 17 cages are spread across the area. Known cage locations include at 51.38, 16.57, at 49.86, 15.56, at 44.86, 14.94, at 44.18, 9.12, at 44.67, 20.73, at 45.41, 15.84, at 38.16, 18.67, at 37.54, 17.96, at 39.2, 15.98, at 39.92, 16.50, at 41.28, 12.49, at 40.89, 11.73, at 47.37, 13.14, and at 46.24, 12.15. Two additional cages exist near those coordinates that are not individually listed.

Since Patch 6.1, Frozen Slaves also count toward the 50 slaves needed and are found throughout the area at spots such as at 40, 20 and at 38, 17.7. Simply damage them to break them free — no Shackle Key is required for Frozen Slaves.

Shackled slaves scattered around the area can also be freed using a Shackle Key, which drops from Taskmasters. Each key frees one shackled slave and then disappears from your inventory. Keys last one hour or until you leave the quest area. Note that shackled slaves working in the area do NOT count toward the achievement — only caged prisoners and Frozen Slaves do.

Nub the Torturer can be found at at 49.8, 15.6 and is a notable enemy in the area relevant to your farming route through the stronghold.