Demon Stone

A Demon Stone is a magical, blood-red gem. It's harmless to humans, and devestating to demons. In most uses, it's a magical item that is used to exert control over a demon. It can be used to force a demon to do its owners bidding, regardless of the demon's own will. It's complete power over the creature with very few limits save for the magical skill and resources of its wielder.
With sufficient work, the stones can be weaponized to contain or weaken a wide range of demons, though at the cost of the stone's near total destruction. They are chemically, almost an allergen to demons, their kryptonite, their silver, when used in the proper form.

Mechanics & Inner Workings

In practice, a Demon Stone is an element of power over nearly any demon. The stone must first be bound to the demon. This can be easy, with a willing or a weaker demon. In the case of a more powerful demon, it can be a battle on par with an exorcism. The creature must be managed while the spell is cast that will link it to the stone. Once a demon is bound, the stone is somewhat like a remote control. It can force the demon to do its owner's bidding. This can be through spells, direct control of the demon, even using the demon's power as another component in a larger magical process.
Once bound, the stone will apply to only that demon. In order for a demon to free themselves, the stone must be destroyed. So long as it exists, it will hold power over that demon. Even if a demon is killed and reincarnated, the stone will still have some lose power over them. In such instances, the stone must be re-bound, but the process is far easier. They will be able to paralyze the demon with the stone, making it an inconvenience while they do the work of binding them.
Spells can be bound over the stone as well, creating protections and allowing it to act almost autonomously, programmed to perform a function on a demon for all eternity.
When not used magically, the stone is able to be used as a poison. Its effects on demons are in line with Silver on werewolves or Holy water on vampires (and demons). It's exceptionally powerful and deadly, in the right hands.

Item type

Magical

Rarity

These gems are rare on the Earth's surface. They have to be found in the pits of hell, where they are far more common. Across the entire US, there are proabably fewer than 1000 such stones in existence at the moment. They are often destroyed by supposed 'exorcists', who believe these stones give the Demons power. The opposite is true, but misleading teachings go a long way these days.