Start this quest in Mines of gloomingdeep. You will get a companion who will follow you anywhere in MoG (unless you get close to Arias and finish the quest).
If you don't take the slave to Arias they will stay with you indefinately. I took mine down to the pit fiend (last week/MoG gear only) for the extra DPS (gave the slave a shield and mace/hammer) and killed the demon with a lvl 7 ranger. If not for her (I took the DE babe) I would have lost (my HP at end of fight...9%)
And, yes, I had an tank merc. The extra DPS from the slave* saved the day.
As a side note, all three of us were whonking on the PF for quite a bit with my tank merc holding agro.....until I cast Flame lick...once. The tank could not peel him off for the rest of the fight, which lasted around 3 minutes. I think that maybe the Pit Fiend really, really, REALLY hates fire, you guys.
EDIT: Slave, not merc.
Edited, Jul 27th 2013 12:26pm by Bijou