Empress of Light

The Empress of Light is a Hardmode, post-Plantera boss fought in the Hallow. While she can be summoned as soon as Plantera is defeated, she is significantly more difficult than most Hardmode bosses and will typically require post-Golem gear. Fighting the Empress of Light is completely optional and not required to progress. However, her defeat will reward the player with powerful weapons.

Summoning
The Empress of Light can be summoned by killing a Prismatic Lacewing, which is a rare Critter that spawns in the surface Hallow during the night. As it is a rare creature, it can be detected by the Lifeform Analyzer. The Prismatic Lacewing must be killed at night to summon the boss.

Like most critters, the Prismatic Lacewing can be caught with any bug net, which can then be released, and killed, by the player manually.

Behavior

 * The boss uses fast-paced multi-color attacks. She has two attack patterns: In the first stage, she will alternate between various attacks in which multiple projectiles home in on the player. In the second stage, she will fire projectiles across the screen in straight lines at various angles.
 * If she is fought during the day, all of her attacks will deal massive amounts of damage (Like Skeletron, Skeletron Prime, and the Dungeon Guardian), effectively one-hit killing the player. She will always drop the Terraprisma if all damage dealt to her was during the day.

Tips

 * It would be wise to defeat Golem first, so the Cosmic Car Key will become available.
 * Not only does the Empress have to be killed during the day to gain the Terraprisma, but all damage she receives must be during the day as well. Any attack from the Empress during the day will instantly kill the player.
 * Consider using a Bug Net to capture any Prismatic Lacewings when you see them, rather than killing them immediately. This will allow you to easily fight the Empress at a time of your choosing once you are prepared. Note that if the Prismatic Lacewing is released outside of the Hallow, it will immediately disappear.
 * While they immediately disappear, it is still possible to kill the Prismatic Lacewing outside of the Hallow. This is done by having a weapon selected, placing the Lacewing from your inventory, then spamming left-click so that you immediately attack, killing it in time to spawn the boss.
 * It is helpful to obtain the Master Ninja Gear before fighting the Empress, as the dash ability, as well as the chance to completely dodge an attack, can be extremely useful during the fight, especially if the player wishes to fight her during the day. What's more, the accessory is obtainable as soon as Plantera is defeated.
 * Be careful using weaponry that deals splash damage or has a high area of effect, as killing the Prismatic Lacewing at day makes it so the Empress of Light will spawn as a much more powerful boss with one-shot-kill attacks dealing an average of 20,000 damage a hit.
 * The Rod of Discord can prove extremely useful during the fight in order to dodge her strong attacks.
 * Since the Empress' attacks can instantly kill the player during the day, it can be used regardless of whether the player will take damage due to the Chaos State debuff or not.

Trivia

 * This boss was described as "a beautiful bullet hell" by Cenx before 1.4 was released.
 * The Empress of Light is one of the two new bosses added in the 1.4.0.1 update, the other being Queen Slime.
 * She and Plantera have the only musical instrument drops of any boss, with The Axe and the Stellar Tune having the same drop chance as well.
 * The Empress of Light is one of 5 bosses with exclusive music, the others being Queen Slime, Plantera, Duke Fishron and Moon Lord.
 * The Empress of Light is one of the 3 out of the 10 hardmode bosses that are not necessary to beat the game.
 * The Empress of Light is most likely inspired by the bullet hell shoot 'em up video game series Touhou Project.