Life-spore

Life-spores were an aspect of the Enemy.

History
The crew of the Narcissus discovered the life-spores inside a lesser species ship on an asteroid. The spores then infected the crew, killing the crewmember James Watt.

However, they quickly became more than just a danger to space travellers when the logic of their presence began to affect the Spiral Politic, leading the asteroid to inconceivably become a colony. One survivor remained, Captain Jennifer Alistoun, who had more fights against the species, both adapting between every encounter. Suddenly, another warlike species, who had no interaction with the "life-spores", began referencing the "life-spores" (who were now one with the Enemy) in their culture.

The Great Houses began to notice the pressing threat of the Enemy's new identity, who had been retroactively inserting themselves into the past, following the passage of events the lead to the Narcissus, finding themselves in the third quarter of the 20th century on Earth.

Their infestation wasn't gaining enough gaining enough awareness, despite them trying various methods of infestation, so in an effort to boost themselves, they inserted themselves into the "meta-flow", becoming iconic. Now they gained the power to go back so far into the past, into the "Old Times", they establish that they were bio-engineered by the "lesser species", now "Creators and Agents"; something which the Great Houses consider an affront to their past, and an attempt to usurp them.

Agent A, who had been thinking this in the morgue containing John F. Kennedy's dead body (who had died from a heart attack, preventing the "life-spores" that grew inside of him from bursting out), in the date of 1967, knows that to prevent this timeline of events, he must kill him to prevent the spores from succeeding in their "false history". (PROSE: The Annotated Autopsy of Agent A)

Behind the scenes
Although not named, sufficient detail is given for readers to identify the creatures which "blossom" out of the life-spores by bursting out of the chests of humans as the Xenomorphs of the non-DWU Alien science-fiction franchise, although it seems to be implied that Xenomorphs are merely a pop-culture representation of the life-spores as opposed to the two species being one and the same. Interestingly, Xenomorphs, or at the very least creatures identical to them, had previously been confirmed as existing in the DWU via cameos in Mindwarp, So Vile a Sin and Dalek.