The Parasite of Mortrex is a unique Tyranid bioform, the result of a Norn-Queen's genetic experiments and tests to enhance its broods of bioforms. Named after its most disturbing ability and the planet where it was first encountered, the Parasite of Mortrex is a bat-winged Tyranid creature evolved from Tyranid Warrior stock. Next to its multiple claw appendages and fanged maw, the Parasite of Mortrex has a barbed tail tipped with a hollow tube, like a snake's fang, capable of implanting its still-living victims with Ripper parasite embryos. Within hours, or even after a duration as short as a few heartbeats, the Ripper parasites mature and will devour the unwilling host from the inside out to fuel their accelerated growth.
The Imperial Fortress World of Mortrex was one of the most heavily defended planets in Segmentum Ultima -– a world where fortifications sprouted from every mountainside and bastions punctuated the landscape. For days, when Hive Fleet Kraken attacked, this formidable defence held back the vicious hordes. However, during the tenth day of the invasion, the Imperial Guard defending Mortrex encountered a winged bioform they had never seen before. Without warning, the monster dove towards the human entrenchments, stabbing at the Imperial Guardsmen with lightning-quick strikes of its tail. The creature's victims were thus injected with Ripper parasites, many of which grew to maturity within a few heartbeats, devouring their hosts from the inside out. Amidst agonised death screams, a swarm of Rippers ruptured through cracked bones and torn flesh as they emerged in a spray of blood. The few survivors told of the new Tyranid threat, a horrifying creature that implants organisms into its still-living prey. They called this creature the Parasite, and all knew that to face it was to risk the most horrific death imaginable. Two weeks later, the planet of Mortrex was overrun by vast, ravenous tides of Ripper Swarms. All that remained was a single Astropathic transmission that warned of the Parasite. Fortunately for the Imperium of Man, it has not been sighted since the battle on Mortrex. Like other Tyranid "experimental" bioforms like the Red Terror or the Doom of Malan'Tai, it remains to be seen whether that Hive Fleet's Norn-Queen will judge the bioform successful enough to warrant its genetic maturation and ultimate full-production, or if it will remain an ultimately unfruitful genetic testbed never to be produced again.
- Codex: Tyranids (6th Edition), pp. 137-138
- Codex: Tyranids (5th Edition), pg. 60