Inside the shadow damage area, there are 6 iron maidens for you to open, Rekarth should be able to lockpick all.
1: Following Ulab's request, near the "tutorial" for the achievement, close to two golems.
2: Near the monument patrolled by a shadow, close to the shadow door.
3: Near the prison cells, close to the aggressive patrolling group of soul eaters/overseer.
4: Near the strapped fecthling.
5: Before you jump down to the corpses, following to the end of the path where you see a vision of Rekarth killing Sendri.
6: Near the Fecthling Reanimator, closer to the stairs to leave the shadow damage area, by the end of the room.
Credit to ALEC for their
guide.