Quake 2 Horde Mode

Sometimes I play Quake 2 (remastered on Steam) and in the deathmatch games there is sometimes a really cool horde mode - it looks like a normal deathmatch level in the "join game" list, but when you play its a coop horde shooter. But quite often its not there to join.

Does anyone know more about this horde mode? I would love to play more of it.