It's a 2vs2 map of Tentacles and other creatures from Xen vs Black Mesa scientists and Gordon Freeman
Tentacle Creatures start with 3 of them in the center, they can't leave the silo unless the doors are breached. It's pretty balanced and fun scenario. No generals though, and only land units. Variety of land strategies can be used. You can find the scenario by searching "Blast Pit" in scenarios tab.