User blog:Super Saiyan 7 Somebody/top seckrit Bloon in BMC NK doesn't want you to know about

It's called Vortex, and is apparently going to make an appearance in some sort of timed event thing like Bloonarius. It's located in a SWF file used by Bloons Monkey City.

Here it is inside the flash decompiler. In the second picture you can see the timer like how Bloonarius has.