Spiderdemon

The Spiderdemon or Spider Mastermind is a recurring high-level monster found in classic Doom, The Ultimate Doom, Doom II and Final Doom. A reworked version appears as the final boss of the 2016 Doom.

Doom/Doom II/Final Doom
The Spiderdemon is not mentioned in the Doom manual, presumably to avoid a spoiler. Perhaps as a result of this, the manual for the Depths of Doom Trilogy erroneously states that it appears in Doom II only.

The Official Doom FAQ (included in Doom 1.4) describes the creature as a 'huge robotic chaingun shooting spider," adding "it will make you wish you never were playing DOOM in the first place. Watch for it at the end of the third episode of DOOM, and numerous times times in DOOM II."

The Doom II manual presents the Spider Mastermind as the mother of the Arachnotrons: "You guess the Arachnotrons had to come from somewhere. Hi, Mom. She doesn't have a plasma gun, so thank heaven for small favors. Instead, she has a super-chaingun. Crap."

Doom (2016)
The Spider Mastermind serves as the final boss of the game, and is implied to be the commander of Hell's forces during the game's events. It is massively larger than before, with a bulkier body and a plethora of new attacks rather than just its original super-chaingun. It is implied to be the same Spider Mastermind fought in E3M8: Dis in the original Doom.

The creature is shown to manifest after Olivia Pierce is pulled down into the ground: this appears to be the process of merging described in the Codex entry, which is necessary for the creature to fully manifest itself. Whether Olivia still existed within the Spider Mastermind after it formed is unclear.

Doom II RPG

 * Main article: Spider Mastermind (Doom II RPG)

In the Doom II RPG, the Spiderdemon Mastermind appears as the game's second boss.

Trivia

 * The Spiderdemon bears a conceptual resemblance to Krang from the 1987 edition of Teenage Mutant Ninja Turtles. Both pilot walking machines with the difference being that the Spiderdemon's machine is a much larger and a more generic-looking quadruped.
 * In the 2012 Teenage Mutant Ninja Turtles series, there is the Kraang Spider Gunner, which seemingly completely parodies the Spider Mastermind.
 * Both the Doom I and Doom II Official Strategy guides identify the Spider Mastermind as male.