Genshoku no Kotou

The story begins with a man's letter. The man finds himself in a cave on a deserted island without memories of his past. The island is a hellish landscape full of monsters competing for survival. There is a huge castle gate with countless human remains around it, and inside the gate are tribesmen who speak a foreign language. The man decides to observe the tribe for the time being, but...
(Source: Sukima, translated)