A_SpawnFly [(string FogActor)]
Checks whether a boss cube has reached its target and if so spawns a monster there. It also telefrags anything in the way of spawning.
If FogActor is provided, the specified actor will spawn in place of the default SpawnFire. Also, the sound that plays at spawn will be the see sound of the FogActor indicated.
This example is taken from Doom's spawn cube.
Spawn: BOSF A 3 BRIGHT A_SpawnSound BOSF BCD 3 BRIGHT A_SpawnFly // See SpawnFire Loop