Prismatic Lacewing

The Prismatic Lacewing is a rare butterfly Critter that spawns in the Surface Hallow during night time after Plantera has been defeated. Killing it in the Hallow during the night summons the Empress of Light boss. Killing the Prismatic Lacewing during the day will not spawn the Empress of Light. As well as this, the Lacewing can be caught with a Bug Net, and be used to craft a Prismatic Lacewing Jar or alternatively, released in the night again to summon the Empress of Light.

Tips

 * It is best to catch it even when you don't want to fight the Empress of Light, instead of looking for it later.
 * You should take any summons off while farming, in case it kills the Prismatic Lacewing.