The Orcish Complex

You can find the entrance to this Dungeon East of Eidos:

Note that you need at least level 15 to access this dungeon.
Once inside the dungeon you will find the prisoners that are needed to be set free for the Orc Prisoners Quest in Hero of Eidos Questline. Make sure to start the Quest before entering the dungeon, or you won't be able to set them free.

Floor 0 of the dungeon and location of Prisoners.

Floor 1 of the dungeon and location of Prisoners.

Upstairs at the end of the dungeon use the exit portal to get out of the dungeon. There is a 5 minute cooldown after you finished your last dungeon until you can enter the next one.
While most dungeons typically require a minimum of two players, this one can be done solo. (Dungeon entrance is free of charge!)