Feminids
Feminids is the name for the Feminists, 'Progressives', and all round socially justice minded types whom are less interested in our nerdlinger hobby once only fit for sun-dodgers, and more into playing a meta game of ideological Space Invaders. They send out a scouting force of Y-Gene Stealers, establishing a foothold, using subterfuge to slowly turn the world of Warhammer 40k to their whims. These activities send out a psychic message through the Immeterium of social media to attract a splinter from Hive Fleet Activistian. Sometimes these Y-gene Stealer cults will attempt to overthrow the societies they parasitise, but unlike their Tyranid cousins they will more often engage in low-level insurgency and growth, until the full Feminid invasion force arrives.
In the best traditions of the Emprah's Imperial Guard, the invasion will proceed slowly, in a methodical forward march, trying to take ground one hobby store at a time, using a marching barrage of Outrage and Offense class artillery rounds, and massed volley fire of Shout-rifles (Noise Marines are sworn rivals, as although they share a niche, there's depths of Chaotic corruption even a Chaos Marine wouldn't stoop to).
Forced onto the defensive, as the Emperor's loyal servants reel from that most alien of phenomena--being outnumbered and outgunned--, the guardsmen (AND WOMEN) desperately dig in, trying to find any foxhole to cover from the shrapnel and blast damage. Perversely for the 40k setting, many will succumb to madness, either willingly throwing themselves into the maws of the Feminids, or attempt surrender, in the desperate belief the swarm will show mercy. Of course being Grimdark, this is sheer folly.
There IS however some hope. Successful defense relies on the Emperor's Inquisition discovering an initial infestation early, followed by massive, unrelenting and final counter-attack. With any luck the loyalist servants of the Imperium can thus save their world from being consumed and left a dead dull mournful rock, with only the distant echos of the departing Feminid fleet screeching praise to their deity. The most corrupting, ruinous of Ruinous powers, Diveerzitee.