this is a boss idea i had for @Megalodon_Prime 's terraria mod he mentioned in some of his boss projects description: this thing is a rare miniboss that spawns in the jungle after plantera has been defeated. it spawns plague mothers that split into plague slimes (both of which spawn naturally) and you can look at them inside the project :D. the minions and the boss itself spawn naturally in the underground jungle. you can spawn it manually using plague sludge (made from 99 plague gel)
Kirby and the Amazing Mirror for the boss theme (this is a remix done not by me, so credit there) if you decide to add this into a terraria mod (or any mod for that matter) ask me and i'll decide whether or not you have permission to use it. i recommend you make a different song for when you use this to avoid copyright infringement (let me hear what song will be used so that way i can decide whether or not it suits the boss)