Roleplaying Adventure
The Heart of Yhtill is an adventure included in the Amazing Adventures rulebook in which the player characters must stop a man possessed by the ghost of al-Hazred from retrieving the eponymous jewel from the Black City of Irem.
Jewel
In the above adventure, The Heart of Yhtill is described as a fist-sized glowing yellow diamond. Its exact connection to Yhtill is unclear.