Complex sentence: Terrigenesis gives Hive a body composed of cellular parasites which survives by inhabiting dead human hosts, gaining their memories in the process, and enabling him to manifest an alien-like appearance with tentacles protruding from his head.
Split into simpler sentences: Terrigenesis gives Hive a body composed of cellular parasites which must inhabit a dead human host to survive. While inhabiting a body, Hive gains the host's memories, while also being able to manifest an alien-like appearance with tentacles protruding from his head.