ضَرَبْتَ أَنْتَ نَفْسُكَ — You yourself struck (someone).
ضَرَبْتَ: past verb; -ta is the attached nominative subject.
أَنْتَ: detached subject pronoun reinforcing -ta.
نَفْسُكَ: nominative semantic emphasis of the subject pronoun; nafs is mudāf and -ka mudāf ilayh.
The object of “strike” is not overtly stated.
The ḍammah, nafsu-ka, marks emphasis of the subject, not an object.