https://en.wikipedia.org/wiki/Baobhan_sith
Back in the early game there was a lot of focus placed on following the Celtic mythology theme. Unfortunately this theme was somewhat killed with the Dragonfire update, but it is thankfully making a return with these new questlines.
This mob was used to drop the rare gems which were required for upgrading the warden weapon to meteoric. Now she sadly just drops pages.
She has two spawns: one to the south of Chillmist and one to the south-west of Fellfire. She also likes to walk a lot.
Other similar mobs: Vedranox, Skydancer, (the blue whisp I dont remember the name of), Otherworld Harbringer, Crystalhorn. They all dropped rare gems.
The Otherworld (Annwfyn, pronounced a-new-fin) is a very major part of Celtic mythology and is referenced in almost every piece of literature on the subject. The creatures and people from the Otherworld often had mystical powers and came to the mortal world to interact with heroes.
If there is any interest, you should read the Mabinogion. It provides a pretty good sense of what the stories were like back in the day and you can relate many game aspects to parts of the tales.
Unfortunately, the tireless and near impossible work required to become powerful in Celtic Heroes is not a theme that can be found in Celtic Mythology. Usually goals were clearly defined and things were accomplished in "a year to the day" not "keep questing and killing and just hope you get good loot".
