Description
According to my research, the nearby Fall Walkers create a vacuum when they die, sucking in any nearby energy. If enough is captured, the Fall Walker is reborn, but the energy source is depleted. This should dissipate the Keystone's magic for good.
Upon completion
It is done then. By destroying the Keystone, you have rid the world of a terrible source of evil. It was important enough for my beloved Arthur to sacrifice his life. Though few will know of his sacrifices, or of yours, I know, and I thank you.