This is a mob that is can pop in any of the "Hot Zones" A regular seeming mob will suddenly become (when getting low on HP - approx 20% or so) - "An Unmasked Changeling" and they will look like whoever or whatever they are fighting at that time (from you, or a pet or whatever).
They drop the augment for that Hot Zone.
Currently in Sebilis it is a
Stone of Might
AC 8
HP 30
Type 7
Solvent type xi
It can spawn anywhere in any hot zone - and the drop will be specific to that zone (based on level of zone).
Enjoy =)
Nethor