Non-mythos novel by Henry Kuttner and CL Moore, first published in 1943. It features the city of Carcasilla which has a vague similarity to Carcosa. Given their connection with HP Lovecraft it is possible that they were influenced in the naming of the city by Carcosa.
The novel involves an alien of the race that had once ruled the city returning with a band of 20th-century humans who had been kept in suspended animation. Although the alien is greeted, it is revealed to have sinister intentions and the 20th-century humans bring about the destruction of the city to preserve humanity in the bleak far future.