A Malanthrope (Tyranicus cadavi terriblis) is an odd creature created by the hive mind to collect specific DNA strands and is found lurking around war torn battlefields.
The Malanthrope is a large serpent-like creature that looks similar to a Lictor. It has feeder tendrils and is about twice the size of the Zoanthrope, another creature it closely resembles. Rarely seen, the Malanthrope follows behind a Tyranid attack, appearing even after Rippers. It selectively collects and processes genetic material from fallen enemies before itself being reabsorbed into the biomass. The creature grasps dead, injured and still-living foes with its long groping tendrils, stuns them with its sting and consumes them, feeding them into its maw with its small but dexterous arms.
The Malanthrope's tendrils carry a venomous poison that is injected through the skin and causes excruciating pain. This poison disables the target and the Malanthrope then absorbs its body into the bloated sacks present on its torso, where the victim's genetic material is slowly extracted and processed -- a terrible death for any individual unlucky enough to be caught still alive by the Malanthrope. After it has processed the victims genetic material, it will carry on this information to Norn-Queens, who will use it to create new strains of Tyranid organisms.
On the tabletop:
It is a Venomthrope raised to the power of Zoanthrope or vice versa also it can be taken in broods of up to 3 as a HQ choice for 110 pts each (90 in the first version of 8th ed, and 140 with the Chapter Approved 2017). They're pretty good is what I'm saying.
Their rules were updated in The Anphelion Project, 2nd Ed. They're now 25 pts cheaper and Elites, so they don't compete with Flyrants. Think of them as having the same relationship to Venomthropes as Warriors have to Termagants: +1S/T, +2W, Save is two better, and add Synapse. They also grant Preferred Enemy to everything in their Synapse range if they're involved in a Combat where you destroy a Unit, and have a bunch of other fluffy rules that will never actually matter. They're now better than "pretty good", and well into the realm of "awesome", sitting just below the Flyrant on many players' lists of good Tyranid Units.