Necrobloon

The Necrobloon is a dangerous bloon, as it can spawn Skeleton Bloons and Zombie Bloons, one of each per 5 seconds, as well as releasing 5 of each of the aforementioned on death, having 1000 HP, and moving as fast as a BFB! It looks like a purple bloon with a black hat and comes at round 66. But that would be boring if it was just that, would'nt it?

Elite Necrobloon
This version now spawns 3x as fast, releasing, instead of Skeleton and Zombies on death, releases Ghoulbloons! Only three of them, lucky for you. Oh, this version has 10000 HP and moves as fast as a ZOMG, coming at round 137. It looks like the first form, only it has a large staff. Oh, it can also attack:

Stave Pound: Uses its staff to deal 3 damage to a nearby tower. Weak, right? Hoever, if it gets the finishing blow on a tower, said tower turns into a Ghoulbloon!

Chaos Necrobloon
But there's still more! If you survive to round 333, you'll be greeted by a bloon that spawns one Ghoulbloon ''per second! ''On death, it releases both 3 Elites and a G.H.O.U.L.! This dangerous version is twice as large and has red veins pulsing through its body. It has "only" 10000 HP, but the speed of a blue bloon! It can attack like last form, but this time it has three attacks:

Stave Pound: Same as last time, only this one spawns 5 ghoulbloons and does 7 damage!

Chaos Bolt: Beware! This shoots a small purple ball in any direction that has a 1% chance to explode each frame. On the explosion, any tower in a 500 px radius takes 5 damage, and 3 Ghoulbloons travel on to the nearest stretch of track! Yikes!

Scream of the Underworld: Its final attack, used on death, the tower closest to the end of the track turns into a Seigebloon!