The Arachnotron is a returning monster that appears in Doom Eternal, and a spawnable demon in Battle mode.
Arachnotrons are built with genetic material recovered from the remains of the original Spider Mastermind, then fused with deadly machinery. While its actual body exhibits limited mobility, the oddly cerebral qualities of the Arachnotron make it a prime candidate for the impulse-controlled prosthetic frame that it now permanently inhabits. The UAC facility responsible for the Arachnotron's creation, intended to engineer and weaponize demons of its own design, was overrun after an outbreak occurred at its assembly plant, killing everyone inside. The facility, still operating under the directive of the administrative AI, continues to manufacture Arachnotrons autonomously.
The design is very similar to the Arachnotron's original appearance in Doom 2, the main difference being that the cannon is mounted on a "tail" extending above the creature rather than mounted on the underside of the chassis. They only fear the doom slayer.
Arachnotrons have three main modes of attack: their preferred method which they will use at medium to long range is fire from their main cannon. The gunfire has a ragged spread that makes it difficult for the Arachnotron to aim accurately, but at the same time tricky to avoid as dodging one projectile might result in being hit by another. The damage from this weapon is surprisingly high.
At closer range an Arachnotron can insert two more attacks into its repertoire: very close it has a leaping melee attack resembling the pounce of a jumping spider, and slightly further it can fire off volleys of time-delay grenades with decent splash damage from a pair of mortar-like launchers that pop out of the sides of its body.
Arachnotrons with intact plasma cannons will occasionally jump to walls or ceilings to use them as vantage points: if their legs are shot while they are on such a surface, they will fall to the ground and be rendered vulnerable for a short time while they right themselves.
Arachnotrons are the most common of the Heavy class demons and the first to be introduced, along with the tutorial for weak points. Destroying an Arachnotron's cannon is vital in reducing the threat it poses, but at the same time an Arachnotron that has lost its cannon will become far more aggressive, rushing the Slayer in a manner more akin to a Hell Knight rather than trying to keep its distance. Destroying the cannon only removes the creature's gunfire attack: it retains its grenade launchers, and these cannot be destroyed.
While all weapons will damage the weak point, the Sticky Bombs mod for the Combat Shotgun, the Precision Bolt mod for the Heavy Cannon and shots from the Rocket Launcher are particularly adept at damaging weak points, with only a single Sticky Bomb or Precision Bolt being needed to destroy the cannon if it is hit directly. If the player feels they need more time to line up their shots, shooting from mid-air with the Chrono Strike Rune equipped is a good idea, particularly when using Precision Bolt or trying to hit the weak point with the Ballista. The Plasma Gun, Chaingun and the primary fire of the Heavy Cannon are the least effective means of dealing damage to the weak point.
Since they are the first Heavy-class demon introduced, most weapons are effective at dealing with an Arachnotron: they can be disposed of extremely quickly with just the Combat Shotgun's fullauto mod at close range. Arachnotrons are somewhat ungainly when turning and constantly dodging behind them is a good way to avoid damage once dashing has been unlocked.
Like all Heavy-class demons they require 3 units of fuel to kill with the Chainsaw: this means it requires some degree of planning to use the Chainsaw to kill an Arachnotron. In general, it is best not to attempt to do so, as even if the player has enough fuel there are far more dangerous targets it could be used on instead.