I'll catch you on the download!
It is a khukuri with a light blue energy blade.
#124: Attack of the Centaurs
Sky uses the khukuri against the Centaurs to help back up the Winx. He strikes down one of the Centaurs alongside Helia and his Rose Thorn Bracelets but incorrectly shouts the weapon's name as Kukri Moon.
Once swung, the khukuri releases a wave of light blue energy that can be used to knock the opponent down.