The Spider Mastermind is the main antagonist of the video game Doom. It was the final boss of the game and was originally called the "SpiderDemon". It's name was changed to the "Spider Mastermind" in Doom II, possibly due to the message at the end of Doom stating "The loathsome SpiderDemon that masterminded the invasion of the moon bases and caused so much death has had it's ass kicked for all time."
The villain itself appears as a large brain with red eyes, a big mouth with fangs and with two small arms on top of a four-legged mechanical base. Once the Spider Mastermind is killed, the level ends, thus beating the game. In Doom II, it appears (extremely rarely) as an enemy in the game, as in, he appears multiple times and it is not necessary to kill it to beat the level.