Black Amazon of Mars - Leigh Brackett

GRIMLY Eric John Stark slogged toward that ancient Martian city—with every step he cursed the talisman of Ban Cruach that flamed in his blood-stained belt. Behind him screamed the hordes of Ciaran, hungering for that magic jewel—ahead lay the dread abode of the Ice Creatures—at his side stalked the whispering spectre of Ban Cruach, urging him on to a battle Stark knew he must lose!

Stark is taking a dying friend, Camar the Thief, back to his native city in the north of the planet. Just before his death Camar entrusts Stark with a talisman he had stolen, wanting him to return it.
Soon, Stark is ambushed by bandits and presented to their leader, the warrior Ciaran.
Joining up with Ciaran’s band, Stark embarks on adventures and battles—through cities under siege and conflicts with strange aliens—that will decide the ultimate fate of the talisman and Camar’s homeland.