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 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 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
If fought 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, 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
For both phases of the fight, the Empress has a scripted set of attacks she will always go through in order; there is no randomness as to what attacks will be used at what time in any given phase.

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

 * It would be wise to defeat Golem first, so the Cosmic Car Key will become available.
 * On Expert Mode and above, the Witch's Broom is a good substitution for Cosmic Car Key, which can be obtained during Pumpkin Moon.
 * For Master Mode, The Black Spot is another option for being available during Pirate Invasion.
 * The Empress has to be defeated during the day (between 4:30 A.M and 7:30 P.M.) to gain the Terraprisma. Any attack from the Empress during the day will instantly kill the player as well as have a yellowish hue.
 * If attempting to fight the Empress during the day to obtain Terraprisma, defense and damage resistance should be completely disregarded in favor of damage and mobility, as any damage taken will be beyond lethal regardless. Additionally, the damage blocking offered by the Master Ninja Gear and Hallowed Armor can allow the player to occasionally make mistakes without instantly dying.
 * 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.
 * 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.
 * In order to prevent accidentally summoning the boss, the Guide to Critter Companionship prevents the player from damaging critters, including the Prismatic Lacewing.
 * 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.
 * Equipping an item that allows the player to dash can prove extremely helpful in dodging her attacks.
 * Don't use Bombs or Grenades, as they will not hit the Empress of Light unless She is on the ground
 * Try defeating Queen Slime a few times, as it will drop the Crystal Assassain armor, which boosts mobility.
 * Use ranged weapons that rain down ammo, like the Daedaleus Stormbow.

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 can be seen as the counterpart to Duke Fishron in many aspects. Both have noble titles, are generally meant to be fought after Golem and are summoned using a critter that can be obtained earlier in Hardmode.
 * The Empress' wings bear a striking resemblance to that of Cenx's Wings.
 * The color of her attacks will change from rainbow to yellow during the day, indicating that the attacks will do massive damage.