M.A.U.L.

The M.A.U.L., or Mega Airship of Uneven power Level, is the secret boss of the M.O.A.B. Madness Special Mission. It takes 1500 hits to destroy and holds an A.E.B. with 1000 HP and a B.B.O.S.S. with 1600 HP. Like the B.B.O.S.S., it is immune to anything that does extra damage to M.O.A.B. class Bloons. It looks like a yellow A.E.B. with sharp teeth in the front, and a M.O.A.B. Assassin bullet (The one from the ability) under the red "no" sign (which is a tower that shoots the bullet) instead of a bomb. It travels at the speed of a M.O.A.B. and has an ability:

Maul: Shoots a tower with the M.O.A.B. Assassin bullet on it, dealing 15 damage. This cannot be avoided; it homes in on the tower like a missile if it moves.

How To Unlock
To unlock this, you have to, in this order: WARNING: If you lose in any step after this warning, or go to a wrong track/etc. you have to start again from step 2!
 * 1) Beat the M.O.A.B. Madness mission.
 * 2) Go to Mount Magma.
 * 1) Get a Master of Fire, 2 Big One Mortars, and a Ground Zero. (You can have other towers)
 * 2) Use the Ground Zero ability. The Master of Fire makes the lava rise by adding more, and the Ground Zero gets more power from the Big One Mortars, making the volcano erupt and popping EVERY SINGLE BLOON that is and comes on-screen (Including Orange Bloons, due to the Ground Zero's Bloontonium) for 20 seconds! (Then, all your towers lose infinity+1 health)
 * 3) Then, go to M.O.A.B. Madness. Round 1 only has 20 M.O.A.B.s, but there's a Round 2! 30 B.B.s appear. (Only 250 HP) The eruption from Mount Magma hit 30 of the 50 M.O.A.B.s, turning them into B.B.s. Beat the 30 B.B.s to find yourself at Drag Strip. (Meant to be a path up a mountain) You get $5000 more and get to place your towers again...
 * 4) Start Round 3, consisting of an A.E.B. with 250 HP, which spawns 2 Dael Bloons, and a B.B.O.S.S. with 400 HP, which spawns 2 Rainbow Bloons. You go back to Mount Magma, get $15,000 and another tower replacement, and the power to start the last round...
 * 5) Start Round 4 to encounter the M.A.U.L.! Have fun!

Reward
After defeating the M.A.U.L., you get 2000 Monkey Money (750 for additional times), 10 Tokens (1 for additional times), 100000 XP (25000 for additional times), and 100 AP (5 for additional times).