Veran the Sorceress of Shadows is used by Koume and Kotake to spread the land of Labrynna as well as its past and present with darkness. By doing so she first possesses Impa, whom Link helps save from being attacked. Later when they meet Nayru, Veran is able to possess her so to be able to travel back in time. Once 400 years in the past she manipulates Queen Ambi to build the Black Tower to help search for her love, but instead possesses the queen and uses the tower for her own means. Later, once Link reaches her, Veran transforms into multiple giant forms including a Bee, Spider, Turtle, and Fairy (which she says is her true form) during their battles. She's also able to summon multiple Dark Links to help her.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.