Does the marriage count as legitimate, and is divorce obligatory, after the wife confessed that a man had intercourse with her in the husband's absence 18 years ago?

If it is established that the wife responded to the aggressor and did not resist, then she has committed a grave sin and a great betrayal, and her claim of coercion is not accepted due to the absence of a clear threat. The husband must first admonish her and remind her of Allah and the gravity of her action. If her remorse and rectitude become apparent after the admonition, there is no harm in remaining with her, and their marriage is valid, especially if she initiated the disclosure of the matter and showed remorse. The husband is advised to forgive and pardon her if she is upright and regretful, especially if there are children, while continuing to monitor and educate her. As for the husband, it is wrong to stop praying and going to the mosque because of this crisis; the Prophet, peace and blessings be upon him, used to resort to prayer in times of hardship, and he must return to Allah and seek help through patience and prayer.