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 is not required to progress.

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 Hallow biome 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
If fought during the day, the Empress enters her "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, instantly killing even the most durable 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.

She will always drop the Terraprisma if all damage dealt to her was during the 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.
 * Most of the Empress' attacks are very hard to dodge, so fighting her will require a lot of mobility accessories and preparation.
 * 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 help dodge attacks, as can the Rod of Discord, especially since its self-damage debuff is of less concern when facing a boss that kills instantly.
 * 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 or during the day, it will disappear very quickly (though with some difficulty, it can be killed if attacked immediately upon release).
 * Although the Prismatic Lacewing can only spawn at night, the Empress of Light can be spawned at any time of day.
 * Ranged weapons that rain down projectiles from above, like the Blizzard Staff, can be particularly effective.
 * 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.
 * If Moon Lord has already been defeated, any of his drops will be incredibly efficient.
 * In the day, her attacks bypass invincibility frames from machines that would otherwise give you infinite invincibility frames, such as a fall damage machine. This is similar to the Moon Lord, requiring special setups to become invincible to her attacks.
 * Getting a "The Black Spot" item in master mode can prove particularly helpful as its unlimited flight in combination with its speed makes running away directly left or right avoid all attacks except: Ethereal Lance Ver. 2, the Dash Attack, and the the straight line of Ethereal lances she can use in the second phase. Turn around when she has just either just finished a Dash Attack or initiated a Sun Dance, and be sure to use a homing weapon as the boss will not be on screen for most of the time

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 long-awaited 1.4.0.1 update, the other being Queen Slime.
 * She and Plantera are the only bosses that drop "musical instrument" weapons. The Axe and the Stellar Tune having the same drop chance.
 * 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 Empress of Light is the last nocturnal boss that can be encountered.
 * 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
 * The Empress of Light is commonly referred to having a similar design to The Radiance from Hollow Knight, though currently there has been no comment from the Dev Team

History
Impératrice de la Lumière