Mithril Golem's Heart is the sixteenth optional quest. It can be obtained by talking to Alma four minutes after completing Fire Golem's Heart. Alma gives the player a hint: One of those who are neither alive nor undead carry the key to the prison of the mightiest of them that is on the land surrounded by water in the center of the world. The player must now travel all over the world and kill golems. All golems except for Fire Golems have a 10% chance to drop the Strange Key. This key can unlock the Strange Cage on Central Islands, which contains the boss of this quest, the Mithril Golem. Upon death, it will drop the Mithril Golem's Heart, which can then be brought back to Alma to complete the quest. She rewards them with 1500 Gold, 4400 EXP and Aero Materia.
Defeating Mithril Golem unlocks the team 'Mithril Golem' in the Island Cup. Likewise, completing the quest unlocks the team 'Rays of Light' in the Ningen Cup.
Mae'chen also sells information about how to find Mithril Golem during the quest until you unlock Stange Cage. It costs 625 Gold and tells you how to find the golem.