it's like... blockland... except you can only use 4x4x4 blocks, and the draw distance is smaller and has black fog instead of white. oh and there's AI monsters and stuff.
wait, random thought, but why hasn't anyone built anything awesome using events to trigger zombie spawns? forgetin' A
yes that part is easy
the whole making-a-scenario-leading-it-into-a-giant-loving-crescendo-event-miniclimax bit is not
If you're prepared to brave eventing you can make almost anything with my bot mod, which I will distribute to some(so I can continue to test and get things rolling)
An average server with low player count(6 players) can handle 30 or so bots at once. Using effective eventing, you can control the number on screen and still make an effective whatever, and support as many players as needed.(however for a co-op thing 4-6 is best)
An entire campaign of L4D could be recreated through use of the same methods they already use.
If you want to beta... I'll be making a topic in mod discussion.
At any rate, back to atomosphir.
It looks great, I'll have to try it whenever it opens up/gets multiplayer.