The Arachni-rig is for all intents and purposes, the lovechild between Doc Ock's armoured limbs and FEAR's REV9 Powered Armour. Basically a heavily modified and more advanced version of the civilian Servo-rigs, these discount Battlesuits are created by Necromunda's House Van Saar.
By itself, a Servo-rig is a mechanical exoskeleton commonly seen in heavy industry, giving their users extra limbs and enhanced strength to aid them in performing manual tasks. So one can imagine if someone used this as a weapon of war. The Arachni-rig is, therefore, the end result of House Van Saar collectively slapping their dicks and tits on a Servo-rig 'till it resembles something like Cyberpunk 2020.
Each rig is an artisan piece and are controlled by the wearer's nervous system or via a direct MIU. They are armed with a twin-linked heavy Lascarbine and four Servo-Arms (Which can be swapped with either a Rad Cannon or a Plasma Gun), which give the Arachni-rigs' wearer a spider-like appearance and each one can be replaced with various weapons. Though it is an advanced piece of machinery, it is not uncommon for Van Saar's gangs to wear an Arachni-rig, in their battles within the Underhive, you know, just in case you want to silently shout to everyone of your micropenis issue.