A Genestealer Primus is a type of Genestealer Cult Hybrid. If the Genestealer Magus is considered as the 'right-hand man' of a Genestealer Patriarch then the Genestealer Primus is the Patriarch's warmaster and chief lieutenant. While it is the role of the Magus to corrupt, the Primus exists solely to lead his cult into war. A Primus only emerges from hiding when the cult is ready to openly act, a war leader whose natural ferocity is paired with the best stolen weapons and wargear the Cult has to offer.
Genestealer Primus as its named implies, are highly skilled combatants in the Cult, second to only the Patriarch himself. The skill and prowess of a Primus is known to best even the most veteran experts of the Deathwatch chapter of the Emperor's most noble Adeptus Astartes. The Primus uses the best weaponry and equipment that has been scavenged, looted from the battlefield, or outright stolen from Imperial armouries by Cult operatives.
The Primus is a commander in times of war, but whilst the cult still remains in hiding, it is his duty to spread the Genestealer Cult to other planets. His innate gift for leadership and coordination sees him militarize the cult in short order, masterminding the takeover of vehicles and even spacecraft. A strong champion and lauded hero amongst the brood, the Primus emerges only when the cult reaches a position of strength. Such individuals appear in an early generation, but typically not until the third or subsequent cycles. The imperative to trigger the creation of a Primus is either when the cult’s numbers reach critical mass, their numbers and influence sufficient to take over a prey world, or when a hive fleet looms on the edge of that planet’s system. The Primus’ particular quirk of the Genestealer life cycle occurs independently, enhancing the development of an especially robust host to produce a largely humanoid warrior of prodigious strength and tactical guile. Where a Magus boasts a mental acuity and telepathic powers the equal of a Space Marine Librarian, the Primus has a supernatural dexterity and surety of focus that can see him bring down adversaries twice his size.
When a Primus is on the battlefield, he will be the one directly issuing orders to fellow Cult members. However like all the key leaders of a Genestealer Cult, if the Primus is killed or assassinated then all cohesion within the Cult may break apart in a panic. This leaves it vulnerable to extermination by any number of hostile institutions such as the Arbites, Space Marines, local Imperial Guard Regiments, competent PDF's, local Inquisition or the Sisters of Battle.