Hags

Powerful magic using shapeshifters.
More powerful when together in their Coven as three of them.
Evil beings.
Resistant to certain damage - standard damage, fire. Not resistant to - Poison, Force, Radiant. They all hold onto a small handsized item when in the Ethereal Plan - possilby what they use to jump in and out? Morgantha has a bag on her belt. The other two unknown. 
Night Hags thrive on corruption and fear.  They are tempters and dealmakers, and consume the souls of evildoers for pleasure and nourishment.  Their skin is harded against elemental forces, and for no mortal metal save silver.  So long as the members of a hag coven remain nearby to one another their arcane power is greatly increased.