What is the ruling on someone who says, "It is forbidden upon me that you do such-and-such," and then does that very thing?

It is not permissible to swear by anyone other than Allah. Whoever swears by other than Allah has committed shirk (polytheism) and must repent. As for one who swears by something unlawful, if he intended divorce, or ẓihār (a pre-Ibn Hajar term for a form of divorce likening one's wife to one's mother), or an oath, then the matter is according to what he intended. If he did not intend anything, then an expiation for an oath (kaffārat yamīn) is incumbent upon him. If he intended divorce, then his wife is divorced. If he intended ẓihār, then he must pay the expiation for ẓihār, which is the freeing of a slave. If he cannot find one, then he must fast for two consecutive months. If he is unable to do that, then he must feed sixty poor people.