They were popularly supposed to inhabit the lesser moon, from which they descended upon Barsoom at long intervals. Where they visited they wrought the most horrible atrocities, and when they left carried away with them firearms and ammunition, and young girls as prisoners. These latter, the rumour had it, they sacrificed to some terrible god in an orgy which ended in the eating of their victims.
I had an excellent opportunity to examine them, as the strife occasionally brought now one and now another close to where I stood. They were large men, possibly six feet and over in height. Their features were clear cut and handsome in the extreme; their eyes were well set and large, though a slight narrowness lent them a crafty appearance; the iris, as well as I could determine by moonlight, was of extreme blackness, while the eyeball itself was quite white and clear. The physical structure of their bodies seemed identical with those of the therns, the red men, and my own. Only in the colour of their skin did they differ materially from us; that is of the appearance of polished ebony, and odd as it may seem for a Southerner to say it, adds to rather than detracts from their marvellous beauty.
But if their bodies are divine, their hearts, apparently, are quite the reverse. Never did I witness such a malign lust for blood as these demons of the outer air evinced in their mad battle with the therns.
All about us in the garden lay their sinister craft, which the therns for some reason, then unaccountable to me, made no effort to injure. Now and again a black warrior would rush from a nearby temple bearing a young woman in his arms. Straight for his flier he would leap while those of his comrades who fought near by would rush to cover his escape.
The therns on their side would hasten to rescue the girl, and in an instant the two would be swallowed in the vortex of a maelstrom of yelling devils, hacking and hewing at one another, like fiends incarnate.
But always, it seemed, were the black pirates of Barsoom victorious, and the girl, brought miraculously unharmed through the conflict, borne away into the outer darkness upon the deck of a swift flier.
Fighting similar to that which surrounded us could be heard in both directions as far as sound carried, and Thuvia told me that the attacks of the black pirates were usually made simultaneously along the entire ribbon-like domain of the therns, which circles the Valley Dor on the outer slopes of the Mountains of Otz.
As the fighting receded from our position for a moment, Thuvia turned toward me with a question.
"Do you understand now, O Prince," she said, "why a million warriors guard the domains of the Holy Therns by day and by night?"
"The scene you are witnessing now is but a repetition of what I have seen enacted a score of times during the fifteen years I have been a prisoner here. From time immemorial the black pirates of Barsoom have preyed upon the Holy Therns.