Magos Cath has performed extensive testing on synapse creatures as they are the lynchpin of his efforts to create an artificial Hivemind. Without the synapse relays found within these creatures control of the "lesser" Tyranids would not be possible. But these creatures present a special complication as they are especially difficult to shield from the Hivemind. They require full shielding of the synapse nodes within the creatures cortex. Magos Cath's will is channeled through the synapse beasts into the other Tyranids in a synthetic Hivemind. The most powerful relay found within the subjects Cath has studied resides in the Hive Tyrants. Of all synapse creatures the Hive Tyrant has the largest and most complicated of all studied synapse nodes, but all synapse creatures are difficult to produce and control. The growth and implantation process for creating a viable Malanthrope is costly in the extreme so they are rare within the Magos Cath's growing menagerie of living weapons.
Here is my completed Malanthrope. Just like the Hive Tyrant I found the perfect bit to use as his head. This was an easy paint. A lot of drybrushing. I was trying to decide whether I wanted to paint this model in bright colors to contrast the rest of the army and to show its toxicity, but in the end I decided to use biohazard symbols instead. It just felt right for Mechanicus.