A spawn cube is a slowly rotating cube with each face of the cube having a skull imprinted on it. It is half of a pair of things that depend on each other to function as part of the Doom II final boss, the other being the spawn spot. When the cube launcher is awakened from its initially dormant state, cubes begin to be launched toward each of the spawn spots on the map, one at a time in the order those spots were added by the map's designer. Once a spawn cube has been launched at the last such spot, the process repeats beginning at the first spot. While the spawn spot targets may be placed at any elevation, the cubes themselves always travel horizontally, due to limitations of the Doom rendering engine.
When a cube reaches its target, it bursts into flames similar to those created by the arch-vile and then vanishes, spawning a random hellspawn (excluding the Spider Mastermind, cyberdemon, and former humans).
On maps occupying the MAP30 slot, if the player is standing close to the target spot when the cube lands, a telefrag results. Invulnerability and the god mode cheat code are insufficient to protect from this.