Lady Eiluned is one of the nine original Sidhe. When she first arrived in Arcadia she went to the Splinter Mountains and talked to the wind there.
She, and by extension her House, has an ambiguous nature. She and her followers took part in the Battle of the Riven Mountain on the side of the Sidhe, but joined only part-way through.
Lady Eiluned was instrumental in giving Boggan a fae soul. As a result of her part in this, Lady Eiluned was changed, so that her appearance was like that of the sun, and she dwelt no longer in Arcadia. She returned there only twice, first when summoned by Lord Aelil and later, after the Sundering, to protect Deep Giant and Snow Daughter on their quest to bring about the reincarnation of the Mountain King.
[And yes, Lady Eiluned did kill Deep Giant in the Battle of the Riven Mountain. She helped the Troll because of the promise she made to the Trolls during the Boggan affair - and the Trolls were screwed anyway, because she had the Emerald. She's crafty.]