The spiderdemon, called the spider mastermind in Doom II in reference to being the entity that masterminded the invasion of the moon bases, is the boss of the final episode of Doom, Inferno, and again of the final episode of The Ultimate Doom, Thy Flesh Consumed. It has 3000 hit points and is one of the toughest creatures in the game. The spiderdemon appears as a huge brain with facial features on it, mounted atop a cybernetic chassis with four mechanical legs and a chaingun, making it similar to a spider at first glance.
The Doom II game booklet humorously presents the spiderdemon 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.
The spiderdemon is not mentioned in the Doom game booklet, presumably to avoid a spoiler. Perhaps as a result of this, the game booklet for the Depths of Doom Trilogy erroneously states that it appears in Doom II only.
The huge chaingun on the spiderdemon's chasis fires three bullets with every shot, much like the shotgun guy's shotgun, but at a rapid rate of 425 shots per minute. When a spiderdemon begins firing, it will not stop until its target moves out of view or dies, or if it is stunned (where it will walk a bit immediately after the pain state) or killed. Similar behavior can be observed in the heavy weapon dude and the arachnotron.
When fighting a spiderdemon, it is essential to keep one's distance as much as possible; even at medium range, the monster's attack can be devastating. Using the more powerful weapons is recommended against it, although since the spiderdemon has a low pain chance, rapid-fire weapons will not completely prevent it from attacking. Using melee attacks against it is extremely dangerous, except in the rare cases where the spiderdemon is pinned by another attacker. In E3M8: Dis, it is possible to stand up on the ledge above ground level and stay at great distance from the monster. At this range, the spiderdemon's attack will not aim toward the player, and therefore never hit its target. Also, the spiderdemon is large and tall enough that, even while standing on the ledge, projectile attacks will reach it.
- Upon death the spiderdemon collapses into a heap, with an accompanying series of explosions. Her mouth, complete with pointed teeth still intact, is visible on her otherwise ruined corpse.
- Near the beginning of Doom II MAP20, Gotcha!, the player is given the opportunity to lure the spiderdemon and the cyberdemon into fighting each other via monster infighting. Unless a third party intervenes, the cyberdemon usually wins. It may be convenient to allow the cyberdemon to win since, while its attack does more potential damage, it is possible to dodge.
- Despite being called a "spider" the monster has only six visible limbs: two arms and four mechanical legs. Incidentally, the Doom novels inconsistently describe four mechanical legs at one point and six mechanical legs at another. 
- The sprites are different from the original latex and metal model . The model has a 3-barreled chaingun implanted right below the mouth, while the game sprites have a multi-barreled chaingun implanted into the base.
- Level 28: The Spirit World in Doom II is the only level designed by id Software that includes more than one spiderdemon at the same time. Two appear near a group of arachnotrons, on the "Hurt me Plenty", "Ultra-Violence", and "Nightmare!" skill levels.
|Shots needed to kill1,2||Mean|| Standard|
health, no armor)
health, security armor)
health, combat armor)
|Baron of Hell||117.61||17.02||100||159|
- This table assumes that all calls to P_Random for damage, pain chance, blood splats, and bullet dispersal are consecutive. In real play, this is never the case: counterattacks and AI pathfinding must be handled, and of course the map may contain additional moving monsters and other randomized phenomena (such as flickering lights). Any resulting errors are probably toward the single-shot average, as they introduce noise into the correlation between the indices of "consecutive" calls.
- The target must be close enough to compensate for the weapon's recoil.
- Assumes that direct hits are possible, which does not occur in any stock map.
In classic Doom, the spiderdemon is first encountered on these maps:
The IWADs contain the following numbers of spiderdemons:
|Game||ITYTD and HNTR||HMP||UV and NM|
- May be encountered earlier if the super secret level is played.