This is the 15843rd most frequent Polish word.
tajemniczych
"Of mysterious" or "mysterious."
The word 'tajemniczych' is used here as an adjective modifying 'schodów' (stairs), indicating that the stairs possess a mysterious or enigmatic character.
Na końcu tajemniczych schodów znajdowała się starożytna komnata.
At the end of the mysterious stairs, there was an ancient chamber.
In this sentence, 'tajemniczych' modifies 'zjawiskach' (phenomena), describing natural phenomena that have mysterious aspects.
Czytałem książkę o tajemniczych zjawiskach przyrody.
I was reading a book about mysterious natural phenomena.
Here, 'tajemniczych' describes 'osób' (people), referring to individuals whose identities or actions are enigmatic or not fully understood.