Alien Swarm with a twist. Better AI, new mutations, new mapper tools.
All alien types seen here can be spawned by the new director, both as members of hordes and as wanderers around the map. The frequency of hordes and wanderers depends on the difficulty settings and the number of marines, and can also be tweaked by mappers.
The frequency of each individual wanderer type can be modified for each map - for example, Sewer Junction B5 has more parasites, mortarbugs, and harvesters than Landing Bay, and the first map of Paranoia has more boomers and shieldbugs.
The director can also spawn multiple NPCs together. This is currently used to spawn 4 shamans in a pack, but it can spawn any combination of alien types and scale health, size, and speed, or even make an alien immune to fire, ice, electricity, or flinching.