Moon Mistress

The Moon Mistress is reffenced twice in the Tal'Dorei Campaign Setting book in the Mooren Lake Region.

Mooren Lake
Centuries of whitestone runoff from the Alabaster Sierrs has allowed deposits of residuum to collect at the bottom of the lake - and the residuum has formed into a thurmming organ of raw magical energy has begun to gain sentience. A pair of twin islands in the center of the lake are blanketed by an ever-present mist, and folk tales tell of a beautiful, ageless woman known only as the Moon Mistress.

From the Quest: Renew the Mists
The Moon Mistress, a night hag who takes the guise of a water nymph....