誰か and 何者か both mean "someone", however I never encountered the latter other than in a novel. Is it a more literary word or is there another nuance ?
何者か meaning "someone" is not particularly literary, but it is stiff and formal, and typically used in criminal or military contexts. It usually refers to an unidentified criminal, intruder, attacker, etc., who is considered harmful. Obviously, ordinary people don't often get the chance to use 何者か in everyday conversations.