The spider mastermind, along with the Cyberdemon, is a boss in Doom. Basically looks like a gigantic brain with teeth on a metal base, which features mechanical legs and a chaingun. Supposedly masterminded the invasion of the moon bases.
The mastermind is the main boss in the third and fourth episodes of Doom, but also has several cameo roles in Doom II, Plutonia and TNT.
It features a chaingun which fires standard bullets (each bullet gives 10 damage at Ultra-Violence). It can easily shred you at short to medium range, but isn't very effective when far away.
The mastermind can be killed by 15 rockets or 2 BFG's. The latter is the reason why he (she?) is so easy to beat in the third and fourth episodes; all you have to do is run up to him and fire a couple of BFG rounds before he can even start firing.
When pitted against each other, the mastermind loses to the Cyberdemon due to the cyberdemon having a more powerful weapon and more HP.
The spider mastermind is also known as the Spiderdemon.