Here is one of my concepts for Bosses in Minecraft has multiple phases, and in each phase has different behavior and forms. Simply because I think this concept is extremely cool and more original than something like "Wither but stronk". Enjoy this article.

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Replicator

Health: 550 HP (275 ❤️) Overall, 180 HP (90 ❤️) in First Phase, 150 HP (50 ❤️) in Second Phase, 200 HP (100 ❤️) in Third and Last Phase.
Speed: Mimic the speed of the Player in First Phase, Immobile in Second Phase, and 50 SP in final phase.
Damage: Varies on Phase, 6-23 HP (3-11.3 ❤️) using its weapons in First Phase, 10 HP (20 ❤️) using scurry and 4 HP (2 ❤️) using its bites in Second Phase, Block toss takes 18 HP (9 ❤️) upon impact, Dash attack deals 60 HP (30 ❤️) if not dodged properly, and Lighting instantly deals 12 HP (6 ❤️) with brief fire damage afterward in Third Phase.
Behavior Class: Impervious.
Drops: Energy Reactor and 1,800 EXP.
Category: Boss, Metamorphic.

  Replicators are extremely powerful bosses that can be found within Abandoned Power Plants rarely found within Igneous Caverns. Igneous Caverns are a type of Cave that naturally generate below -0, are the home of a few NPCs, Mole people, and Diamond Golems, and form around the outer core of the world. Abandoned Power Plants are among the many structures that can only generate within these Caves, have a 2% chance of doing so, and are made up of 5 parts: The Entrance, The Lobby, the Northern Module, the Southern Module, and the Central Zone. The Central Zone is filled with various experimental creatures within the broken capsule, and a Replicator has a chance to emerge from a broken capsule. There can only be one Replicator at a time in a single world, and their presence is marked by the ground touched by them turning to a silvery, gray metal.
  Replicators in the first phase take upon the appearance of the Player but are entirely Gray in color and attack using their variants of Swords, Crossbows, Cannons, and Hammers, and attack any non-Replicator Mob in sight. They have the ability to absorb EXP and Heat, and are damaged by weapons that are of the same type it currently uses at any given moment. Once 180 HP is depleted, the second phase begins and the Replicator transforms into an array of much smaller, nano-bots that resemble Silverfish, and swarm around to infect various areas with its Gray Goo. Weapons with Flame Touch, Lava Buckets, and Flame Throwers are the only way to damage the Replicator in this phase, and the Player has a chance at being inflicted with slowness during this phase. In its final phase, the Replicator shifts to a vaguely, humanoid robot with the ability to toss infected blocks around, strike nearby mobs with lighting, perform dash attacks and slam the player with its arms, and can now be damaged by any weapons of at least diamond tier. After being defeated in its final phase, the Replicator would drop a rare block called the Energy Reactor that can store EXP for later use, can transfer EXP Orbs to power various redstone and farming mechanisms, can convert/revert blocks, and can create mob hybrids of two or three different mobs.

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Notes:

1. Replicators do not need to be fought for Game Progression, they are intended as an extra challenge for those who already killed the other bosses I would later get to in future posts multiple times, would like means of storing EXP and amping their Redstone creations, and are fans of Hybrid Mobs.
   
2. Trust me, the Ender Dragon and the Wither are going to be stronger than this thing, I want to put the Strong in Bosses.
3. Replicators are based around the common trope of escaped lab subjects and Gray Goo nanomachines.
4. The Replicator's personal name if it had one, similar to the Wither being "Simmons?" and Ender Dragon being "Jean?", would be "Morgan?".