Soul Sand

Soul Sand is an agent bought for 25 Monkey Money. It can only be placed on the track, and only on land. It slows all bloons that go over it by 20% for the rest of the game. It slows down S.T.E.V.E.s even more. The Pro version is unlocked after 50 uses (1250 MM total) and puts ice under the Soul Sand, so it slows the bloons by 35% and even more for S.T.E.V.E.! The Epic Version is unlocked after 50 MORE uses (1250 more MM, 2500 total) and is now a 1-way door. The bloons get popped by 3 layers trying to get through, which takes 3 seconds, though blimps fly over the door. (The slowing still works for all Bloons)

Trivia

 * Another obvious reference!
 * This is the lowest-costing non-joke agent in Bloons Conception Wiki.