Demon Seed is a variant of the arcade game Phoenix. It's a shoot 'em up where the player has to take out wave after wave of demons. Controls involve moving left and right along the bottom shooting at the demons and using a shield to avoid colliding with them or their projectiles. The game contains five different waves. The first two involve groups of bats that fly around in different formations and dives towards the player. In the third and fourth waves there are instead seeds flying back and forth. These will soon hatch into winged demons which has to be shot directly in the head to be killed. Hitting a wing will only remove the wing which will soon grow back.

In the fifth and final wave a demon attack ship will appear. To destroy it, it's necessary to first break through its underside and fire at its rotating shields. Once a hole has been made the player has to hit the gargoyle that is residing in the centre of the ship. Meanwhile there are bats that attempt to crash into the player's ship that have to be fended off. The ship will continuously descend and eventually crush the player if it reaches the bottom.

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