The Spellsong was an evocation spell granted by Eilistraee to her speciality priestesses, the Sword Dancers. It was a form of Spellsong, a type of magic that used song and sometimes dance to draw power from the Weave
The Sword Dancers could sing and dance to channel the power of Eilistraee. They had the ability to sing their magic even amidst the chaos of the battle, while dodging and dancing around or while wounded.
Multiple priestesses could join in a song and dance around a focus (which could be a creature to aid, a fire, a longsword with its blade planted in the ground or a holy symbol of Eilistraee), invoking magic of greater power.
A single priestess could use the Spellsong to invoke magic equivalent to any 4th (or lower) level divine spell, or to return a single 4th (or lower) level spell cast through another Spellsong in the last two days to the mind of the priestess, to provide some healing to injured targets, or to grant spell immunity.
When more than one priestesses joined the ritual, the healing magic of the Spellsong was empowered, and other effects could be obtained. A group of seven or more priestesses could replicate the effects of the spells cure blindness, slow poison and cure disease; a choir of nine could dispel magic or remove curses and a group of twelve could even summon magic equivalent to the spell regenerate.