Empress of Light

The Empress of Light is a Hardmode, post-Plantera boss fought in The Hallow. While she can be fought as soon as Plantera is defeated, she is significantly harder than most Hardmode bosses and will typically require post-Golem gear. The Empress of Light fight is optional and is not required for progression.

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 between 7:30 PM - 12:00 AM once Plantera has been defeated in the world. As it is a rare creature, it can be detected by the Lifeform Analyzer. The Prismatic Lacewing can be killed at anytime in any biome to summon the boss. Note that outside of the Hallow biome, the Prismatic Lacewing will disappear nearly immediately, requiring the player to use persistent or quick weapons to kill it.

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

Behavior
If fought or summoned during the day, the Empress enters an "enraged" state, indicated by the color of her attacks turning from various colors of the rainbow to a bright yellow. While enraged, all of her attacks do tens of thousands of damage each hit, one shotting even the most well equipped players. Additionally, If night falls once more and she is still not defeated, she will vanish and drop nothing.

If fought in Expert or Master Mode, she will always use her enraged attack pattern, but without the massive damage increase (unless it is daytime). She will also not vanish if left alive for an entire day.

Attacks

 * Prismatic Bolts - Summons a cloud of prismatic bolts that home in on the player after a short time. These bolts do not have perfect tracking, and will miss if the player makes a sharp turn as they approach. If enraged, extra bolts are created, and they will spread out more before homing onto the player.
 * Dash Attack - Positions herself to the left or right of the player before charging horizontally in the player’s direction.
 * Sun Dance - Hovers above and slightly to the left of the player and emits 6 rays of light (8 if enraged) that slowly rotate clockwise. 3 sets of rays are created per sun dance, each positioned slightly more clockwise from the last.
 * Everlasting Rainbow - Releases a ring of 13 stars that spiral out clockwise, then spiral back in the same direction. These stars leave behind a trail of light that will damage the player.
 * Ethereal Lance - Summons a swarm of swords that launch in the direction the player was headed when they were summoned, telegraphed by colored beams. These swords are summoned behind where the player was moving, or all around the player if they were stationary. The swords do no damage until launched. If enraged, extra swords are summoned, and they will launch faster.

Attack Pattern
The Empress has set attack patterns for both of her phases.

First Phase
When spawned, the Empress will hover in place for a few seconds before executing the following attacks in order: This sequence of attacks will repeat indefinitely until the second phase starts.
 * 1) Prismatic Bolts
 * 2) Dash Attack
 * 3) Sun Dance
 * 4) Dash Attack
 * 5) Everlasting Rainbow
 * 6) Prismatic Bolts
 * 7) Dash Attack
 * 8) Ethereal Lance
 * 9) Dash Attack
 * 10) Everlasting Rainbow

Second Phase
When the Empress is below half of her max health, she will disappear for a second then reappear above the next estimated location of the player before executing a new set of attacks, with slightly shorter attack intervals than before: This sequence of attacks will repeat indefinitely until she is defeated.
 * 1) Ethereal Lance Ver. 2: Summons four evenly spaced waves of swords that travel across the screen, telegraphed by colored beams. The first wave travels from the left side of the screen to the right, the second from right to left, the third wave from top-left to bottom-right, and the fourth wave from top-right to bottom-left.
 * 2) * If enraged, there will instead be six waves of swords that target specific points around the player instead of traveling from one side of the screen to the other. The first wave travels from the left side of the screen to the right of the player, the second wave from the right side to the left of the player, the third wave from top side to the bottom-right of the player, the fourth wave from top side to bottom-left of the player, the fifth wave from the bottom side to top-right of the player, and the sixth wave from the bottom side to top left of the player.
 * 3) Prismatic Bolts
 * 4) Dash Attack
 * 5) * If enraged, she will cast Ethereal Lance after the dash before resuming her normal pattern (targets where the player is rather than where they will be, or do nothing if the player is stationary)
 * 6) Everlasting Rainbow
 * 7) Prismatic Bolts
 * 8) Sun Dance
 * 9) Ethereal Lance
 * 10) Dash Attack
 * 11) Prismatic Bolts (Cast in a circle around the Empress instead of a cloud directly above her)

Tips

 * To avoid accidentally summoning the boss, use the Guide to Critter Companionship, which prevents the player from damaging critters, including the Prismatic Lacewing.
 * A Rod of Discord, dropped from Chaos Elementals, will help you dodge some of her attacks.
 * If attempting to fight the Empress during the day to obtain the Terraprisma, defense and damage resistance should be completely disregarded in favor of damage and mobility, as any damage taken will be beyond lethal regardless.
 * The damage cancelling effect of the Master Ninja Gear, Brain of Confusion, and Hallowed armor can allow the player to occasionally take hits without instantly dying.
 * The dash ability from Shield of Cthulhu, Tabi, or Solar Flare armor can increase mobility.
 * Using wings with hover capabilities, such as the Hoverboard or Betsy's Wings, can be quite effective for dodging due to their high speed and acceleration - especially when combined with the Soaring Insignia.
 * 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 and place of your choosing once prepared. Note that if the Prismatic Lacewing is released outside of the Hallow, it will disappear very quickly.
 * Although the Prismatic Lacewing can only spawn at night, the Empress of Light can be spawned at any time of day.
 * Use weapons that are capable of dealing damage reliably from a long range, such as the Blizzard Staff, Possessed Hatchet, or Tactical Shotgun (with Chlorophyte Bullets).
 * She is not immune to any practical debuffs. This can be a good extra source of damage, especially with the use of Ichor, Betsy's Curse, or Daybroken.
 * In the day, her attacks bypass invincibility frames from machines that would otherwise give you infinite invincibility frames, such as a fall damage machine.

Trivia

 * This boss was described as "a beautiful bullet hell" by Cenx before 1.4 was released.
 * She and Plantera are the only bosses that drop "musical instrument" weapons, with The Axe and the Stellar Tune having the same drop chance.
 * Although the Magical Harp is a musical instrument weapon, no boss drops it.
 * 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 Hardmode bosses that are not necessary for game progression. The others are Queen Slime and Duke Fishron.
 * Several sound effects used by the Empress of Light appear to be samples of the "Whale Probe"s communications from Star Trek IV: The Voyage Home.
 * As stated by Crowno, the 'mouth' of the Empress of Light's sprite is actually her nose.
 * The Empress of Light bears a resemblance in both appearance and attack pattern to The Radiance from Hollow Knight.
 * However, Crowno has stated that the Empress is not based on The Radiance but is mechanically inspired by Touhou and other bullet hell games.
 * The Empress of Light possesses the highest health of any female boss in the game.
 * Additionally, she has the 3rd highest overall health of all the bosses in the game, behind the Destroyer and Moon Lord, and the 2nd highest health of any single enemy, surpassed also by the Destroyer.

History
Impératrice de la Lumière