A classic work of science fiction.
The Earthman Carse, walking through the night of a seedy Martian town, is followed. Which leads in turn to an offer of the title sword. Which, he knows, means the one making the offer knows where the legendary tomb is, and goes to loot it, forcing the would-be seller to come. It holds a black bubble-like sphere, of the ancient science of Mars, and Carse is too careless, and plunged into the depths of history.
It involves the ancient seas of Mars, the difficulty in piloting them, winged beings, a haughty princess and galley slaves, a war, and more.