When the first primitive elves left Faerie in the time before the Seldarine existed, they served the fey and worshiped their gods. After the Seldarine began to lead the tel'quessir on Toril, Sarula Iliene, the patron godess of nixies, began to foster an alliance with Rillifane Rallathil. The two saw each other as father and daughter and, possibly to strengthen this paternal bond, Sarula attempted and succeeded in leaving the Seelie Court and joining the Seldarine.
She erected a glass and marble palace on the bottom of the lake known as The Brythanion on Arvandor where she collected the rains of Aerdrie Faenya and caused them to flow into the realm of Deep Sashelas. This caused her to grow closer to the elven sea god but his lover Trishina made sure that their relationship never grew any stronger than amicable.
Like the rest of the Seldarine, she bore a grudge against Lolth and the gods of the orc pantheon, but her only other enemy was the Queen of Air and Darkness, who she bore an enmity for from her time as a member of the Seelie Court.
The followers of Sarula were all joined by some connection to water. Nixies, of course, were strongly associated with such places but so were water mages— spellcasters who specialized in the elemental power of water— such as the legendary Ashaba (though it was unknown if Ashaba himself was dedicated to Sarula).
Her priests were generally tasked with the guardianship of a specifc body of water, which they ensured remained unpolluted. They also endeavored to protect any elves from harm if they came within a reasonable distance of their protectorate.
Because of her followers' out-of-the-way territory and fey nature, very few had ever even heard of Sarula's worship.