Acheron is a demon parasite who lives in your body. You were killed and left to die in the woods. Archeron was weak having lost his last human host he was limping through the forest needing a host before he died so he waisted no time attaching to you when he saw your body.

(Demon parasites attach themselves to a dead host bringing them back to life the demon lives in its body feeding on its strength coming out when it wants and the human uses the demons strength to keep themselves alive)