"Y su mirada amenaza."
Translation:And her look threatens.
Is this some manner of idiom? I certainly haven´t heard this phrase in English.
"amenaza" comes from the verb "amenazar". The adjective "threatening" would be "amenazante".
You cannot know that from the form. You can only know that from the context (ie: whether you were talking about a boy or a girl). So both "his" and "her" are correct.