Demon Attack on YouTube by Marlin Lee.Marooned on the ice planet Krybor, the player uses a laser cannon to destroy legions of demons that attack from above. Visually, the demons appear in waves similar to other space-themed shooters, but individually combine from the sides of the screen to the area above the player's cannon.
Each wave introduces new weapons with which the demons attack, such as long streaming lasers and laser clusters. Starting in Wave 5, demons also divide into two smaller, bird-like creatures that eventually attempt descent onto the player's cannon. Starting in Wave 9, the demons' shots follow directly beneath the monsters, making it difficult for the player to slip underneath to get in a direct shot.
http://www.youtube.com/watch?v=HaNcNHhKNxA