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 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 Parasite of Mortrex was first encountered on the Imperial Fortress World of Mortrex in the Ultima Segmentum. The solid Imperial Guard defences held back the Tyranid invasion for days, until on the tenth day when the Parasite of Mortrex first attacked. Each victim it claimed added to a swelling horde of Rippers and soon only armoured units dared to leave the fortress' walls. However, even tanks could not withstand the massive numbers of Rippers. Two weeks later, the planet was completely overrun by vast swarms of Rippers. A single astropathic transmission was sent out as the planet fell, warning of the existence of the Parasite of Mortrex. 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 sucessful 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 (5th Edition), pg. 60