Starmasters' Gambit

Original title: Le gambit des étoiles. Translated into English in 1973.

As colonists penetrated the galaxy, a series of strange legends accumulated about the worlds just beyond the rim of our exploration. These legends told of vast black citadels built by pre-human intelligences that dominated certain deserted planets. And the legends agreed that these colossal structures were not only impenetrable to explorers – but were still in some mysterious way activated. This is the story of Jerg Algan, into whose restless hands fell the key to the citadels. This the the story of Jerg Algan whose fate it was to be a "knight" on a cosmic chessboard, leaping from planet to planet as a gambit – a chess sacrifice move – to check the dark monarch who ruled the farther half of the Milky Way.