The Hallow Jar

The Hallow Jar is an ancient artifact from the Primal Age. It was created by a powerful wizard when darkness threatened to overtake the land, and he was able to trap it within the vessel with the help of an ancient dragon, who gave its life essence for it. The ritual that they undertook caused the boundary between the material plane and elemental planes to crack, reopening the Elixien, Lutum, Cindrax, and Precipt Rifts to the Planes of Air, Earth, Fire, and Water. But with the creation of the vessel, he was able to seal the darkness (The Infinite Void) away, preventing it from wreaking havoc and destruction of possibly the entire planet. Satisfied, he hid it away, deep within the Urwald Jungle in a tomb that is only revealed when the elemental rifts have been closed -- or, at least diminished.
Now, millennia have passed and the true nature of the jar has been lost to all. Some believe it never existed. Some think it gives a powerful boon to any who wield it. None know of the true evil that will be unleashed if it is let loose. The Children of the Hallow are among those who believe it grants great power to the possessor, and they have made it their lives' goals to obtain the vessel and unlock its secrets. They are close to discovering how to retrieve it, and they are working on ways to complete their mission without the general population knowing. However, they know not what they are about to unleash.
The Jar is a rather plain brown porcelain with depictions of an amorphous black creature in various arrangements, each of it destroying a town. A thin handle is at each side near the top, a dragon head adorning each. A lid is magically sealed at the top, preventing it from opening. Down the front runs a story in Celestial telling a tale of darkness sweeping over the land and despair taking root in the hearts of the townsfolk, of horrible evil pervading their minds and making them relive nightmares. It ends with a warning: Any who holds the Jar holds the fate of the world in their hands.
The Jar can only be opened by a 9th level dispel magic, after which the Infinite Void will burst free and immediately attempt to possess whoever freed it. They must succeed on a DC25 Charisma Saving Throw or become possessed as the Infinite Void takes over their body. If not removed within 10 days, the body is destroyed. Regardless of if it is removed forcibly or by choice, it will grow to full strength at the end of 10 days. After it is freed and before 10 days have passed, it is in a weakened form. In this weakened form, it is possible to destroy the Infinite Void permanently. However, once it reaches its full strength, nothing short of Divine Intervention is able to permanently destroy it.
Sufficiently skilled magic users are able to return the Infinite Void to its prison by casting the Imprisonment spell with the Jar as the special component.