S4·E24 The Day of Justice

The Winx girls return to Tir Nan Og with the Wizards of the Black Circle, but they end up in the fight of their lives when the magical kingdom is in danger. Nabu makes the ultimate sacrifice to save the kingdom and dies in the process, causing Layla to seek revenge for his death. In the end, she leaves the Winx Club and joins Nebula's army.