noun
Used when the identity of a person is unknown or deliberately unspecified, often with a nuance of mystery or suspicion.
何者かが窓から侵入したようだ。
It seems someone broke in through the window.
彼は何者かに尾行されていた。
He was being followed by someone.
誰か is the common word for 'someone' in everyday speech. 何者か is more formal or literary, and often implies an unknown or suspicious person.
Compound of 何 (nani, 'what') + 者 (mono, 'person') + か (ka, question marker). Literally 'what person?', used to refer to an unspecified or unknown individual.