Genestealer Magus

And lo the Magus proclaimed, "Let there be bitches and hoes!"

And deliver they did.

A Genestealer Magus, sometimes simply called a Magus, is a high ranking person in a Genestealer Cult, the 'right-claw humanoid' of a Genestealer Patriarch and one of the ones that looks the most 'human'. A Genestealer Magus acts as the 'chief psyker' of a Cult, and they use their psychic influence to manipulate and cast away the authorities of the Imperium.

Overview

The Magus plays a vital role in infiltrating a civilized society. Due to his (or her, there's even a female Magus model now!) intelligence, charisma, appearance, and understanding of the society being infiltrated, they are well suited to their role as mouthpiece of the clan to the outer society. The Magus is a special mutation which appears in a clan if a psychically-active species, such as humanity, is infiltrated. As a Genestealer Hybrid of the 4th generation, they appears nearly human, although is invariably bald and heavy boned (which the Magus can just bullshit as "being born that way"). He is highly intelligent and possesses formidable psychic ability. Their "normal" appearance allows them to operate as the public spokesperson for the clan or cult, which is disguised as a legitimate religious, political, or social movement — a role which the purestrain Genestealer Patriarch cannot take.

Magi are always born from at least one parent who is a Psyker, and this, combined with its xenos DNA, makes the creature a highly potent psychic being. They can use their psychic abilities to coerce or enslave others to their cause. Should a Patriarch or Magus dominate a region completely, the psychic shadow that gathers around them will grow darker and thicker until it begins to coalesce. This ever-shrinking miasma hardens into a many-limbed form known as a Genestealer Familiar that will faithfully serve its Magus.

The Magus is second in importance to the Patriarch; under these two individuals there is little hierarchy or leadership. Therefore, both the Magus and the Patriarch are the primary targets of Imperial Assassins. Killing both of these individuals leaves the Genestealer Cult leaderless and in disarray, allowing a military force to follow up and purge the mass of fanatics.

Magi are rare in correlation with how rare Psykers are in the parent population, so a Genestealer Cult is drastically more dangerous on a planet with more Psykers around.

Tabletop

In 8th Edition, the Magus is the Genestealer's cheap psyker option, with one power, one deny, and up to two familiars. The Magus also grants a lesser version of Deny the Witch to all GENESTEALER CULTS units within 6", which they can only use on powers targeted directly at them. It is relatively important to not forget they're affected by his/her own aura, so they can use that free Deny on things that target him/her. You can still only try to Deny each manifested power once, though. Nevertheless, this power is specially designed to make Grey Knight players cry as all of your units can Deny their Rites of Banishment. To be fair, the Grey Knights are just bad in general in 8th so it doesn't really matter.

The Magus is also a pretty important support unit. For Aberrants, bringing a Magus with Mass Hypnosis/Might From Beyond would make these guys become either really hard to kill or insanely strong in CC with S14. For an added 12pts, you can give your Magus a psychic familiar to act as a meatshield bodyguard.

Although a Magus should generally avoid direct combat, when absolutely necessary they can still kick some teeth in with their Force Stave, Cultist knife, and obligatory cult-issue autopistol.