This is the 17398th most frequent Russian word.
темных
"Dark" (genitive plural or accusative plural of "тёмный").
The word 'темных' is used here as an adjective describing the plural noun 'коридорах' in the genitive case, modifying it to convey the trait of being dark.
Она шла по темных коридорах старого замка.
She walked through the dark corridors of the old castle.
In this sentence, 'темных' describes 'лесах', which is plural and in the prepositional case, attributing them qualities associated with darkness.
В темных лесах ночами бывают странные звуки.
Strange sounds can be heard in the dark forests at night.
Here, 'темных' modifies 'сторонах', a plural noun in the prepositional case, indicating metaphorical darkness referring to aspects of the soul.