This one is geared to D&D 3.5, but I think it's interesting.
The Demon's Eye
In the ancient times, before the nine hells were solidified there existed a pit of pure, black nether. Bodiless forces of anger and hatred filled the voids and essentially gave rise to the demons and devils. When the nine hells took shape, the powers of evil were forced to do so as well. Among the first and most powerful to form were the Balors and Pit Fiends. Thereafter followed the lesser essences which where eventually enslaved and formed the progenitors of the lesser demons and devils. However, the most powerful forces resisted the tides of physical limitation, but eventually succumbed and became the most awful demons of the pit, Bataans (CR 22 cousins to Balors outlined in a homebrew book I'm working on).
The Bataans were few in number, but their destructive force was far beyond that of their weaker siblings. These beasts roam the outskirts of the nine hells crushing anyone or anything that happens across their path. Unlike other demons they do not seek to rule over others or to tempt mortals. Their one and only end goal is total ruin.
One such Bataan of particular power took pleasure in making demons and humans destroy one another. With this thought he channeled all of his malice and malevolence into his right eye. This act utterly consumed the demon of all but the strength to place his eye in the mortal world. There it took on the form of a small ruby coiled in gold trimming.
One day, a hapless adventurer stumbled upon the jewel. When he picked it up he felt an enormous surge of power. What little magical ability he had grew ten fold. He was able to perform what some thought were miracles. He eventually returned to his home town to find that a war had started with the neighboring state. Immediately he set to using his new found powers to defend his city.
This adventurer was able to single handedly turn the tides of war and decimate his enemies. Some saw this power as a divine sign, while other found it to be the work of evil magics. The adventurer was slowly outcast by a growing population in his town, while yet another faction rallied behind him. One day a thief came and attempted to steal the jewel, but was caught in the act and destroyed by the adventurer. The adventurer felt betrayed and immediately set to quell those who disapproved of his gift. In days the town broke out into a total civil war. In the end the adventurer was victorious.
The adventurer had vanquished those who opposed him, yet he felt remorse. Having killed his own townsfolk he swore to discard the jewel. However, with one hand outstretched, holding the jewel over a well, the adventurer tried with all his might, but could not let go of it. With one hand he tried to pry the jewel out of his hand, but could not let it go. In desperation, he severed the hand holding the jewel. Feeling a sense of victory over the demonic gem he picked up and examined the amputated hand. This time, he found he could not let the hand holding the Demon's Eye go.
One curious citizen observed the situation and spread the word of what he had seen. Those who supported the adventurer quickly turned on him after hearing of the gruesome spectacle. They rallied and attacked him. One by one they all died at his hand as he fought to survive.
Shortly thereafter a group of traveling soldiers came upon the ruined town and found the lone man amongst the ashes of the city. They summoned the nation's army to put town this rogue mage. Being overwhelmed by the numbers of the military the adventurer was killed, dying with hate in his heart for the country that betrayed him. To his last breath the clung to the jewel, trying to use it to destroy those who turned on him. Afterward, a righteous soldier picked up the gem, swearing to end the madness it had caused. Fearing the power in the jewel and the potential that this soldier might end up the same as the adventurer the other soldiers tried to take the gem from his temptation. The righteous soldier knew the power of the jewel, so he fought to keep it from his weak willed counterparts. The soldier swore to keep the gem from those who would be swayed by its power, so he turned it on the others. In the end the soldier stood amongst the ashes of a fallen army. He said to himself, "only I have the heart to withstand this evil thing, so I will destroy everyone who might take this from me..."
The Demon's Eye
This jewel allows a spell caster to cast spells as though they were four caster levels higher than they are. A non-spell casting player may cast spells as a 4th level sorcerer. Any player holding the Demon's Eye must make a Will save (DC 10) to set the jewel down. Each day the caster holds the jewel increases the save DC by +1. Any evil aligned creature within 100 ft of the Demon's Eye has a 15% chance to know of its presence. Any character that sees the Demon's Eye must make a Will save (DC 10) to resist trying to take it. Every day a character sees the Demon's Eye or knows the individual holding it increases the DC by +1.