The White Guardian

According to legend, Hephaestus, smith of the Gods, wrought a stone of unparalleled power that he presented to the Greeks after their victory over Troy.
Since ancient Theseus broke the eternal stone into four and gave its pieces to four Guardians, history has unfolded at the whim and command of those who wield the stones' energies.
All of the original Guardians have been killed - all save one. Only Corbo, the old Roman gladiator, remains to restore a precarious balance to the ongoing struggle for possession of the stones, and to do this, he must steady the scales of power in not one but several time periods. To make matters worse, the chosen Guardians of the present age, Thaddeus, Raphael, and Violet, are rank beginners.
In The White Guardian, Corbo faces a Nazi threat to England centuries before World War Two erupts, and Raphael begins a search for his mother, Corbo's protegee Guardian, Elena. Raphael's quest will entangle the rest of the Guardians and incidentally determine the fate of an English village that's trapped in a realm where vengeful witches hold history in limbo. Raphael and Thaddeus, using the stones' power and the more subtle magic of music, must find a way to free Elena from a terrible inter-dimensional spell that ensorceled her.