The Sword of Rhiannon is a science fantasy novel by American writer Leigh Brackett, set in her usual venue of Mars.

He is approached by Penkawr, who attempts to coerce Carse into helping him sell the Sword of Rhiannon, which is a relic stolen from a Martian tomb.

The Sea-Kings are engaged in war with the Empire of Sark, descended from the Dhuvians, and both sides wish to obtain the artifacts in Rhiannon's tomb.

The presence in Carse's mind reveals itself to be Rhiannon himself, seeking atonement for the crimes he committed eons before and the last, desperate struggle to save the planet reaches its conclusion as the whole world changes around them.

Rich Horton reviewed the Ace Double edition as "really wonderful pulp Sword and Sorcery, pitch perfect, beautifully written, twistily plotted.