Fey elemental

Fey elementals, also known as "fairy butterfly things", were a type of monster found in Ginaron. It is unclear if they were created by the Ytarans as part of their experiments into planar travel or found somewhere in the Feywild.
They have
DR/5 Cold Iron
and are Incorporeal which means that only magic attacks could hurt them and even then it only had a 50% chance of working.
They can also do direct mental attacks in melee range, which don't require an attack roll, the victim has to make a DC 14 will save and if they fail the victim gets 1D6+3 damage (save for half?)
They also had the ability to cast Sleep on opponents and then possess their sleeping forms. A use of this ability to possess DiCorso's Steward led to the death of the penultimate survivor of the expedition, leaving the Steward alone until his rescue by The Party.
If hit by electricity they also could be potentially stunned like the Fel Elementals.