Progressive Bosses makes the Wither and Ender Dragon much harder, lets you summon stronger versions of them and turns the Elder Guardian into a real fight with an actual reward. Boss difficulty scales with the number of players nearby. For Forge and NeoForge, Minecraft 1.18.2 - 1.20.1; requires InsaneLib.
Expect more health, new abilities like Wither charging and the Dragon respawning crystals, minions for both bosses, extra crystal cages and faster skulls and acid balls. Group boss fights get the challenge they deserve on a Minecraft server with this mod added in one click from the panel.