A Blade of Ages is a powerful sword, capable of more than an ordinary sword. They are described by Seraph as being, "the tools of the gods, distributed among mortals." Only two people have ever been known to wield a Blade in our world; Rhys Grey and Xenophus. But Xenophus is only able to use one because of his habit of absorbing his alternate selves. Two others can wield the Blade, but they are rumored to be dead. The Blade of Ages is unique in being the only object that is present in ALL universes.
The first form of the Blade of Ages seen is the Great Cross. But each and every living thing with a proper spirit has a form associated with them (that includes worlds).