Morph Mod (1.21.2, 1.21.1) (or Morphing Mod) allows the player to Morph into any mob after killing it. You can cycle through all the morphs you have and you can become anything at any point. It is the ultimate disguise. This also is compatiable with any mod that brings in a living entity, so if you added orespawn per say it will work on the bosses themselves. The way you morph into mobs is so unique, it takes each model piece from a mob and dissembles it and resembles it onto the player.
Source: Morph Mod (1.21.2, 1.21.1)
Check Out: Minecraft Mods
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