The Villainess Wears an Idiot's Mask is a Manga/Manhwa/Manhua in english language, Shoujo series, english chapters have been translated and you can read them here. You are reading chapters on www.mgeko.cc fastest updating comic site. The Summary is

A great mage acting as an idiot to get revenge On the day of an unwanted wedding, Winsty awakens as a great mage and is resurrected from the danger of being poisoned. She gets in a contract relationship with the duke of a rival family in order to stop the constant attacks of his parents and her cousin who harmed him while aiming for the position of the lord. She wears an idiot's mask to regain honour and rights.