Upon completing one of the Ancient Dead contracts. The text should probably read either ‘Not one thanks you, naturally.’ or ‘No one thanks you, naturally.’
I must concede that apparently ‘not a one’ is used; I’m not sure even for the poetry reason to create a jarring effect it is appropriate here, if the mercenaries are not surprised that no one thanks them.
The ‘a’ just seems to be a waste of a syllable, when it could be shorted to just ‘not one’ or emphasized with ‘not a single one’.