Although not making an appearance in the distant past, the Primus is a welcome addition to the Cults in the present day. Effectively, the Primus is the melee what the Magus is to the psychic phase. A specifically grown (or emerged) hybrid designed purely for strategy and acts of war.
The Primus has a reasonable profile apart from the Toughness (which remains at 3). His equipment is very reasonable with a combination of bonesword claws, toxins, needle pistol and so forth. This makes his a bit of a ripper in close combat and gives him something to play with at short range as well.
The real shining light here though is the bubble of hatred that the Primus can project. In concert with automatic look out sirs and attached to a large blob of hybrids, the Primus can cause real and significant damage to most things in the game.
Toughness is a problem. But remember that he has unquestioning loyalty to offset this. Plus the Primus is very cheap. At least compared to the Space Marine HQ selections that he can take down…
The only option here is whether to give the Primus a relic or not. Personally, I think this one is about optimum if relics are anyway near tempting:
Primus with Sword of the Void's Eye (90 points).
With some of the other buffs available from other components of the army (e.g., Might from Beyond), this Primus is really able to take down most things in the game (or at least threaten them). Combined with the usual genestealer play style, this could be very deadly.