Prismatic Lacewing

This butterfly is the summon for the Empress of Light and it is the only one. There is no crafted summoning item. You find the critter, you kill it yourself, and the boss wakes up where it died. It cannot hurt you, it will not fight back, it does not regenerate, and other enemies cannot touch it.

It spawns on the surface Hallow only, after Plantera is dead, in the window between 7:30 PM and midnight. It is fussy about ground as well: hallowed grass that has not been mowed, pearlstone, pearlsand and pink ice all count, and nothing else does. Only one exists at a time, so clearing the one you can see is what allows the next. In a Remix world the time restriction is dropped and they appear at any hour. If your world has no surface Hallow at all, Holy Water or blue solution in a Clentaminator will make one.

The game files it as an enemy rather than a critter, which is useful. A Water Candle and a Battle Potion raise its spawn rate, a Blood Moon raises it further, and a Bloody Tear lets you force one whenever you like. Build a platform a good distance above the hallowed ground and sit out the window with a candle up, and several will show. If ordinary enemies are eating the spawn cap, spread sentries and long reaching minions such as the Sanguine Staff around to keep it clear. A Lifeform Analyzer will tell you when one is nearby, which saves a lot of squinting.

A Bug Net picks it up and it keeps in your inventory indefinitely, so you can take one now and release it once the arena and the potions are ready. Released outside the Hallow it vanishes almost immediately, but there is a moment first, and dropping it straight onto a standing damage source such as a Clinger Staff or a Rainbow Gun beam kills it inside that moment.

The kill has to be yours. A trap or another enemy finishing it off on its own does nothing, but if you damaged it first, something else landing the last hit still counts, and a falling star counts as your damage too. You cannot hurt it from a distance, and it renders translucent to tell you so, which is what stops a stray shot from starting the fight offscreen. If you are running a farm and do not want the boss yet, the Guide to Critter Companionship keeps them safe from you, and the mistake everyone makes is forgetting to turn it off when they finally do want the fight.