Future bosses

The future bosses, also known as the Soul Artifact bosses, were a group of three post-Moon Lord bosses that the Calamity Mod added to the game. They were buffed versions of previous bosses, which took the form of their future selves had they not been defeated by the player. They were all summoned using Soul Artifacts crafted with Cinderplates and Exodium Clusters, both of which were found on planetoids.

Ancient Doomsayer
The Ancient Doomsayer was a more powerful variation of the Lunatic Cultist, summoned using the Eldritch Soul Artifact. It performed the same attacks as its weaker incarnation, but with greatly increased strength, speed, and amount. It was able to use all of the attacks in its arsenal regardless of its current health, including Ancient Light, which lingered in the air for far longer and moved much faster, and Ancient Doom. Its fireballs and Prophecy's End attacks were much faster as well. The clones it summoned were identical to the Ancient Doomsayer in appearance, the real one only differentiated from them by the presence of a health bar.

The Devourer of Universes
The Devourer of Universes was a more powerful variation of The Devourer of Gods, summoned using the Dimensional Soul Artifact. The Devourer of Universes was always in its final form, and used its laser wall attacks throughout the entire fight, during which it would always be visible even when invulnerable. The boss's speed had been increased dramatically, and its laser wall attacks were more frequent, more random, and had less room for error. Regardless of the world's chosen difficulty, contact with The Devourer of Universes' head would have instantly killed the player.

Quotes

 * "DOES IT HURT!?" (When the player gets hit by its head)
 * "YOU FOOL!" (When the player gets hit by its head)
 * "WHY DIDN'T YOU DODGE!?" (When the player gets hit by its head)
 * "SO WEAK!" (When the player gets hit by its head)
 * "NO! I AM THE ONE ABOVE ALL! I WILL CONSUME YOUR UNIVERSE!" (When the Devourer of Universes drops to 25% life)
 * "YOU CAN'T BUTCHER A GOD!" (When attempting to instantly kill the boss with another mod's butcher function or an overpowered weapon that deals damage more than 1/2 of it's health)

Dragon God, Yharon
Dragon God, Yharon was a more powerful variation of Jungle Dragon, Yharon, summoned using the Godly Soul Artifact. He began in his first subphase and transitioned to his third subphase at 98% health, summoning Infernadoes, spewing fireballs, and flying at extreme speeds. He remained this way for the majority of the first phase, before transitioning into his teleporting fourth subphase at 30% health. His second phase began in its most difficult pattern, and continued to be as such for the remainder of the fight.

Aftermath
Killing any of the bosses caused a status message that read "Ethereal power courses through you." to appear, permanently buffing all players in the world with these stats: This occurred only the first time a future boss is killed. The buffs would stack for every unique future boss killed.
 * +3% damage
 * +3% critical strike chance
 * +3% minion knockback
 * +3% movement speed
 * +3% melee speed
 * +1% damage reduction
 * +10 maximum health
 * +20 maximum mana
 * +3 defense

Trivia

 * Each of the future bosses had their own themes:
 * Ancient Doomsayer's theme was You Shall Know No More, which was composed by the artist energykid.
 * The Devourer of Universes' theme was UNIVERSAL COLLAPSE, which was composed by the artist DM DOKURO.
 * Dragon God, Yharon's theme was Roar of the Jungle Dragon, which was also composed by DM DOKURO.
 * This theme was fully voiced with its own lyrics by DM DOKURO himself. The song was a warning from Yharon to his master, Yharim, about the strength and unstoppability of the player character. The song's lyrics are considered non-canonical to the mod's official lore.
 * The three bosses had respective individual traits, according to their respective item's tooltips:
 * The Ancient Doomsayer's was knowledge.
 * The Devourer of Universes' was power.
 * Dragon God, Yharon's was loyalty.
 * These bosses were a reference to the "Dream Bosses" from the game Hollow Knight.
 * All of the future bosses could not be instantly killed, such as by using another mod's butcher function.
 * Attempting to do so with The Devourer of Universes would have made a unique message appear in the chat; this would not happen with Dragon God, Yharon or the Ancient Doomsayer.