The Matter of the North
One of the greatest magical materials yet discovered, Mirakite is a dark mineral with incredible magic-amplifying properties: the only other substance of similar amplificative powers being the Golden Apples.
No natural source of Mirakite has yet been found, and its production eludes modern alchemy. Ancient Arranumma possessed a great many Mirakite objects: including immense doors made of the material. Whether they knew the secret of its production, its source, or merely how to locate it, such knowledge is lost: all that is known is that Mirakite will, very rarely, fall from the sky. There is at least one site on the Moon where it falls with some regularity- though still infrequently and in small quantities.
Mirakite remains the only material known to be able to slay immortal beings (or at least those beings made immortal through the use of the Fire of Life )