In the end, Sebastian infiltrates Alfea with Beatrix’s help, kidnaps Sky, and forces Bloom to bargain for her powers and Sky’s life. The Winx girls, plus Beatrix, return to save the day and kill Sebastian, restoring the power he stole from every fairy. Sadly, Beatrix dies in this process. The portal to the Realm of Darkness is open, and Bloom decides she most close it from the inside.
Source: https://www.distractify.com/p/fate-the-winx-saga-sebastian