This is the 4673rd most frequent Polish word.
najemniczej
The Polish word "najemniczej" means "mercenary" (adjective, feminine singular/plural genitive, or dative case).
In this sentence, 'najemniczej' is an adjective modifying 'armii,' showing that the army mentioned consists of mercenaries.
Grupa najemniczej armii przeszła przez granicę.
A group of the mercenary army crossed the border.
Here, 'najemniczej' is an adjective describing 'duszy,' emphasizing the mercenary characteristics attributed to the soul.
Opowieści najemniczej duszy fascynują wielu ludzi.
The tales of a mercenary soul fascinate many people.
In this context, 'najemniczej' qualifies 'naturze,' focusing on the self-serving characteristic of the nature being described.