Moonshae Trilogy
The Moonshae Trilogy was a series of three fantasy novels written for Dungeons & Dragons by author Douglas Niles, and have the distinct honor of not only being the books that introduced the Moonshae Isles into the Forgotten Realms canon, but being the first Realms novels ever written. That's right, these books predate anything involving Drizzt! ...Although people have claimed that originally the Moonshaes were meant to be their own, entirely separate D&D world and it was later hodge-podged into the Realms, ala Kara-tur, but hey, whatever.
The trilogy consists of three books; Darkwalker on Moonshae, Black Wizards and Darkwell, and it revolves around the Darkwalker War; a conflict in which a band of adventurers must lead the peoples of the Moonshae to battle Kazgoroth the Beast, an aspect of Malar bitterly opposed to the Earthmother, a benevolent power of nature who is either a local aspect of Chauntea or one of the Primal Spirits with a particularly close connection to the fey, before he can corrupt the isles into a twisted hellscape.
It has a sequel series called the Druidhome Trilogy.