This is the 12367th most frequent Polish word.
mrocznych
"Dark" or "gloomy" (genitive plural form).
In this sentence, 'mrocznych' is used as an adjective to describe the noun 'corners' (zakamarkach), indicating that these corners are dark, eerie, or shadowy.
W mrocznych zakamarkach lasu kryły się tajemnicze stworzenia.
In the dark corners of the forest, mysterious creatures were hiding.
Here, 'mrocznych' modifies 'times' (czasach), referring metaphorically to a period that was grim, unenlightened, or challenging.
Słuchaliśmy opowieści o mrocznych czasach średniowiecza.
We listened to tales about the dark times of the Middle Ages.
In this usage, 'mrocznych' describes the noun 'mysteries' (tajemnic), suggesting an aura of intrigue and somberness in the mysteries being described.