“The dark entity left its mark on the haunt of its thrall. Cast the light of a black flame upon the castle map.”
Your treasure hunters should now know the identity of the murderer because when a black light is shined upon the map, his/her haunt bears the name of the specter.
Knowing the name of the specter, they can calculate its gematria value (see Quest for the Murderer pt. 6) and open the first lock on the chest.
Knowing the identity of the murderer, the treasure hunters know whose charms (see Quest for the Murderer part 7) hold the digits to the second lock on the chest.
What’s inside? Why, the heart of the murderer of course. This quest is complete. The jar containing the heart will be used in the Quest for the Castle Treasure. (See also how to make a blood message.)