It’s a small metal box, with a lock, and slit in the top and note attached that says “do not open”
It‘s hiding place was shown to Percival by the demon lord of memory, when Percival tried to summon Polk, as per the demon and Polks agreement.
Inside was the key to unlock the box, and 4 envelopes. One with nothing written on it, and the other numbered 1-3.