Iron Dice appear in the Harabah Block of Tartarus, from floors 202 to 213. They may appear in groups as large as four, making them ideal for gaining EXP quickly before the party tires.
Persona 4
Iron Dice appears in halls 5 through 7 of the Steamy Bathhouse within the Midnight Channel. It will typically Stand By until it is the last remaining enemy on the field and heavily damaged to cut its life into pieces and use its Last Resort, damaging everyone on the field. Upon being defeated, it may drop the Pure Iron Lump material. Selling 8 Pure Iron Lumps at Daidara will unlock the Zweihander weapon for the protagonist, which has the highest Attack and Hit rating for the protagonist's weapons available in the Steamy Bathhouse along with a small Endurance boost.
Persona 4 Golden
Iron Dice appears in more places in halls 1, 2, 5 to 7, 9 and 10 of Steamy Bathhouse. They may also be summoned by Wealth Hand. Upon defeat they may drop Smoky Quartz for use at Shiroku Pub.