Revenants are a strange bit. The one above, isn't quite the one we'll see in the comic, but close. They're a combination of parasite and vengeful undead. The small parasite infects the host while they're still alive, lying dormant in their body until death, where they immediately consume and replace the nervous system of the host, spreading tendrils through their body until they have control of the corpse.
During that time, while tapping into the host's brain, they are usually overtaken by whatever anger or vengeful memories their host had most, driving them to kill, almost as if the host's spirit uses the parasite to get its revenge after death. After the target of the dead's rage is killed, the parasite is able to take complete control, spreading even more tendrils out and into muscle fiber, increasing the mass of the host as well as growing a carapace-like skin. If left alive for too long they will feed on any living being in an area, changing as they do, turning more and more monstrous, until there's nothing left of the host and the parasite is fully grown.