The Magic Goes Away — Larry Niven

ONCE THERE WAS MAGIC IN THE WORLD...
But hordes of selfish, short-sighted magicians have used up the mana that moved the world, and the magic goes away. Even the most powerful spells are fast becoming futile, and so Orolandes, sad Achaean with half a sword, goes on a quest in search of the lost power.

But all the gods and Fair Atlantis are dead. The creatures of spirit, the unicorns and centaurs, are dying. Soon all the sparkling things will be gone from the world and only clay will remain -- and those damned stupid barbarians with their damned stupid swords will win after all...