Karl Edward Wagner 
Bloodstone [EPUB ebook] 

Kane. The Mystic Swordsman becomes the living link with the awesome power of a vanished superrace. In the dark swamp where toadmen croak and cower, slumbers a secret relic of the days when creatures from the stars ruled the Earth. In the booty captured in a savage raid, Kane discovers a ring, a bloodstone, which is key to the power that lies buried, inactive but not dead, within the forest. Now Kane, whose bloody sword has slashed and killed for the glory of other rulers, can scheme to rule the Earth – himself!