I've changed it a bit now, but the main concept was that this was more a McGuffin level of weapon for when the players are at a high enough level that it's more balanced - I think the aging effect handles like a vorpal effect, in that it can insta-kill. But by limiting it to non-magic users, and effectively just warriors, it removes the worst abuse (de-aging for an infinite number of wishes) and makes the de-aging more of a threat.
I was looking more for a very strong magic item - after all, it is made from a dragon's powers combined with the powers of the mage who killed him! I prefer a fast aging process because it instills a deep fear in people - imagine being a young elven fighter, who gets stabbed once by an assassin and is suddenly old and grey!
The problem with slower aging is that it's not as useful in a straight-up fight. An insta-kill on most foes when you crit is about the same level of power as a vorpal weapon, and an issue players might hit is that it will actually make dragons stronger! A fitting final act by a powerful dragon, wouldn't you agree? :) Go to Comment
Instead of summoning something when failing, it could trigger a number of traps (the floor falling away, and a pit opening up; a number of arrows/spears being fired into the room; or even spikes coming out of the floor). Go to Comment
"It is said that among those people they have a loathsome custom- that they keep a spotted dog always waiting beside the gate of the yard where they bury the dead, and that in every funeral they allow this spotted dog to feast upon the dead, so that it grows fat and wise with the knowledge of the dead... Many necromancers do seek out these spotted dogs, and ask of them sciomantic knowledge, or take them as familiars." -Author unknown, "The Ways of the Necromancers"