Ahdin's coin of misfortune

This spell is placed onto a single gold coin which appears to be completely normal to any observers once finished. As long as the caster stays within 120 feet of the coin it will remain stable for up to an hour. Once an hour has past or the caster drops concentration as a free action the coin will detonate into explosive flames instantly. The flames fill a 5 foot radius sphere centered on the coin and any flammable objects within that range that are not being worn or carried will ignite. Any creature caught in the range will have to make a Dexterity saving throw or take 4d4 fire damage, or half as much on a successful save. They must then roll 1d20, and on a roll of 4 or lower they will also ignite; taking 1d4 fire damage at the end of their turns each round until they or another creature takes an action to extinguish the fire. However, if the caster and the coin get more than 120 feet away from each other before the coin explodes, then the magic on the coin will fade away harmlessly.

At higher levels. When you cast this spell using a spell slot of 3rd level or higher, the initial damage increases by 2d4 for each slot level above 2nd.