A book that I'm currently writing called Surge features an enemy faction called the Degenerates that are heavily inspired by the Scythians (Indo-Iranian horse nomads that ruled the Eurasian Steppes and Central Asia from the 10th century BCE to the 3rd century CE) and consists mostly of humans parasitized by a prehistoric endoparasite called Echidna (Aengapentastomumn corruptor), which parasitizes and radically alters the entire physiology of a wide variety of organisms from the phylum Chordata.
An excerpt from an in-universe field manual that was given to the protagonist (written in the vein of a scientific dossier or military briefing) implies that Echidna is a pentastomid (an endoparasitic crustacean that feeds on vertebrates) that first appeared during the tail end of the Cambrian period. While most pentastomids evolved to infect and enter a vertebrate’s lungs via an intermediate host, Echidna forewent this reproductive process by tunnelling into an animal’s brain and hijacking their body for the purpose of giving birth to thousands of larval Echidnae.This was a simple, yet ingenious survival strategy since Echidnae possessed no natural defences and an organism controlled by an Echidna would be used as a living suit of armour that allowed the parasite to sustain itself in a hostile environment until it reached sexual maturity.
Echidna originally evolved to parasitize marine arthropods inhabiting Cambrian seas. Upon locating a suitable host, Echidna would use its razor-sharp teeth to cut through the hard exoskeleton without damaging any internal organs and burrow into the host's brain. Upon locating the frontal lobe, Echidna would spread root-like tendrils that fused with the host's central and peripheral nervous systems, bringing the creature under the parasite's control. Once this was done, the host's physiology would undergo a series of changes as the Echidna "edited" the host's genetic structure by adding, removing or substituting parts of the genome with genetic material acquired from previous hosts (e.g the excerpt mentions that the fossil of a parasitized trilobite was discovered possessing raptorial appendages like those of Kodymirus vagans and extremely long, hollow spines that possibly contained venom.) It was also not uncommon for Echidna to absorb genetic components from their hosts using Horizontal Gene Transfer, which would've been incorporated into their offspring's genome.
It wasn't until vertebrates migrated onto land during the late Devonian era that Echidna made the jump from arthropods to vertebrates and truly began to thrive. As countless species came and went, Echidna evolved so rapidly that it was almost impossible for the fossil record to keep up and its genome became a vast catalogue of genetic information compiled from countless extinct species. Over time, Echidna gradually evolved to infect humans that settled on the Australian continent, some 65,000 years ago before the events of Surge.
But for reasons unknown, Echidna suddenly vanished from the fossil record. It wasn't until the early 21st century that Echidna returned from the brink of extinction, when human activity awakened a colony of Echidnae from their millennia-long slumber. The Echidnae wasted no time in undergoing an abrupt re-evolution in order to parasitize not just anatomically modern humans, but also Australian vertebrates that evolved in the wake of the Quaternary extinction event.
Now, to the main event. The stuff I need to know what sort of adaptions would Echidnae need to:
To enter through a vertebrate's ear canal and tunnel into their frontal lobe, without killing the host
Successfully avoid triggering an immune system response
Assume full control their hosts' behaviour and bodily functions
Alter the host's physiology and genome using Horizontal Gene Transfer