What is the ruling of Sharia on entering a room after swearing not to enter it? And how much expiation is required from one who swore not to go to forbidden places and then broke his oath, especially since he said: "If I break my oath, I owe an expiation for every step I take towards the forbidden"?

Whoever swears by Allah not to do something, then does it, has broken his oath (hanith) and is liable for an expiation (kaffarah) for breaking an oath. The expiation for breaking an oath is binding on whoever swears by Allah, or by one of His attributes, or by the Quran, not to enter a room, then enters it without being compelled. The sin accrues from failing to render the expiation, not from the breaking of the oath itself. Going to forbidden places is unlawful by original Sharia ruling, so it is not permissible to break an oath made to abstain from them. The statement (If I break my oath, I owe an expiation for every step I take towards the unlawful) is an oath of obstinacy (nadhr lajaj), and its maker has the choice between fulfilling it or offering an expiation for breaking an oath when the condition occurs, and the expiation for such an oath is the expiation for breaking an oath. One expiation is due for entering the room and one expiation for going to forbidden places.