Swarm of Bloons is a Special Mission that is pretty hard. It takes place at Star Shrine, the hardest track of the whole Blooniverse. You will start with 5000 cash. All will be Red Bloons until Round 6. Every 5 rounds, the layers of the bloon will increase until MOAB. This will have 260 rounds. Also, every round 50 bloons are added.
All bloons from the Evolution Map (except Nuke & Bacterial) form this mission. Bloons that can't evolve further (MOAB, Shock, Hypno, Gold, Super, Orange, 2-faced) will go to MOAB directly (except for MOAB (again?!)).
This will depend on your luck. Well, you have 1000 lives. However, this is the hardest mission.