Nest is a one mission campaign used in Alien Swarm: Reactive Drop mod(RD) for showcasing the power of the "Red drones". In RD these drones can't be ignited, frozen, electro stunned and have a significant health boost which makes map quite challenging. This map, however, can be used in Alien Swarm too, but scary Red drones will be usual drones, just red colored.
I play this map on ,,vanilla" Alien Swarm and it really good campaign!