What is the expiation for swearing by divorcing one's wife by making her unlawful until the Day of Judgment if one returns to smoking cigarettes, knowing that one intends to quit and has begun to cut back?

If the situation is merely an engagement without a legal marriage contract, then the fiancée is considered a stranger (non-mahram), and it is not permissible to declare her unlawful. However, if a legal marriage contract has been concluded, then she is a wife from whom whatever is permissible for a wife is permissible, and the declaration of unlawfulness (tahreem) depends on the intention of the husband.

If the husband intended dhikr (a form of divorce by likening one's wife to a forbidden relative) by the declaration of unlawfulness, then it is dhikr. If he intended talaq (divorce), then it is talaq. If he intended an oath, or to urge himself to do or refrain from something, or if he had no specific intention, then an expiation for an oath (kaffarat yameen) becomes binding upon him.

So, if you had contracted marriage with your fiancée and conditioned her unlawfulness on smoking, then if you do not return to it, nothing is binding upon you. But if you return to it in the intended manner, then an expiation for an oath becomes binding upon you if you intended to prevent yourself from smoking and did not intend talaq or dhikr.

However, if you had not contracted marriage with her, and you intended by the declaration of unlawfulness to prevent yourself from smoking, then nothing is binding upon you, because declaring a non-mahram unlawful is void (laghw), unless one intended dhikr from her, which is upheld by some scholars, and kaffarat dhikr becomes binding if he marries her before consummation.

You must immediately quit smoking due to its immense harms.