"It"

"That thing, it's horrifying. But whatever happens, we can't let it through. Throw everything you have at it! STOP IT AT ALL COSTS!"

- Hamilton, upon encountering "It"

"It"
"It" is the final boss of Bloons Tower Defense Siege. "It" was formed from the remains of all the bloons that have been popped throughout the whole game. "It" is a huge packed ball of rubber, ash, metal, and other things.