This knife is decorated with a gem as the pommel, which looks like a sun from one side and a moon from the other. The blade is a solid white shimmering material (that looks pearly), about 10cm long, and does normal knife damage for most of the time. The hilt is made of a dark wood, wrapped with gold wire. However, it has the weight of a broadsword and can't be used by anyone able to cast magic spells (whether it's divine magic or arcane magic).
When it hits a target for a critical hit, it ages them by 1d6 centuries - this will kill most creatures outright, except for long-lived races (elves etc. may survive, depending on age and dice roll) or races who aren't killed by age (dragons and other similar creatures definitely will survive). If the target is aged by 600 years (i.e. a 6 is rolled), the wielder is also de-aged by 1d10 years. This will affect their ability score adjustments and can even undo other magical aging. However, if the wielder becomes too young, they will revert to being a child, a baby, or may even die, meaning the weapon is not without risks.
The blade is a dragon's tooth, and the weapon was created by a great mage who defeated the dragon. However, the dragon's will is still alive in the blade. If the wielder dies through de-aging, the dragon (a Red dragon named Aranath) is reborn at the same time (and more importantly, in the same place), and will be 1d20 * 30 years old. He will attack anyone around him, as the many years of not-quite-death have broken his mind, and die a true death in 2d4 hours.
There is an ancient saga told about the battle, which ended the dragon-threat amongst the 'Folk of the Plains' as they call themselves. They maintain that the mage was only able to defeat Aranath because the gods of the mountains aided him, and gave him the materials for the Knife of Eternity. He fought the dragon once, but was forced to flee when the dragon overpowered him. However, he was able to take with him a single tooth he knocked out of the dragon's mouth. He used the gifts of the mountain gods to create the Knife, and then enchanted it with his most powerful magics. He knew he would have to win the next fight, before the magic in the knife ran out.
The battle raged for three days, and the dragon was about to finally destroy the mage who had come to kill him, when he threw the dagger with the last of his strength. It pierced the dragon's heart, and his body crumbled to ash, which was swept away by the wind. The mage collapsed in front of his fallen foe, and died.
The people of the plains found him there, and gave him their highest honours. They buried the knife with him, because they believed the dragon's power still slept within it. And who knows, they may be right. Aranath's death definitely changed the magic inside the dagger.
This weapon may be best given to a foe, and then when the players fight him, he accidentally summons the dragon and dies. The knife will become a normal knife if the dragon is summoned, and cannot be re-enchanted.