In the simplest terms, the (often referred to in-game as the "Illuminados Pendant" or simply a "Key Item") is a hexagonal, stone-like artifact used as a security pass. However, reducing it to just a "key" does a disservice to its design.
If you are playing through the game today, taking the time to read every blue leather journal and blood-stained memo offers a deeply unsettling glimpse into the mind of the infected, making the survival horror experience all the more impactful. resident evil 4 codex