Topic category: Minecraft mod ideas exchange
This is for anyone who doesn't know what abilities to give their boss. Here is a list (mostly for beginners)
Minions: When player hits boss or vice versa, chance to spawn [insert mob here]
Cloning: When boss hit has a chance to spawn visually identical entities with low health.
Teleportation: Pretty basic
2nd stage: you can use a "when entity dies" global trigger and do "if entity is (sub) type of [your boss] spawn a [2nd stage version]." You can just make a visually identical mob with stronger abilities.
Fire Trail: on entity tick update turn block at X - 1, Y Z into fire, or make your boss immune to fire and just do XYZ, and shazam, you have a fire trail.
Healer Minions: just do the minion procedure but have the minions "hostile" to the boss but have their attacks inflict instant healing so they actually heal the boss.
I will add more if I come up with them!