What awaited our protagonist who was summoned to another world as just an add-on for the "saint" was a forced marriage with a bird man who exuded poison form head to toe?

Rui Kanzaki is summoned to another world alongside her twin sister. While her sister, who has aptitude for all kinds of magic, is treated as a "saint," Rui, who has no aptitude for magic and only has "poison resistance," is treated poorly. One day, Rui is forced to take her sister's place as a bride to the emperor of a neighboring country rampant with beastmen. The emperor is a man who has the blood of a poisonous demon bird, and is himself known as the "Emperor of Poison," a man whose entire body is covered in deadly poison.

A "political marriage" between a lonely king and a substitute saint. Their fates, which should have never crossed, gradually begin to intertwine.
双子の妹と共に異世界へ召喚された神崎琉衣。
ありとあらゆる魔法に適性がある妹は「聖女」として扱われるのに対し、魔法適性は皆無で「毒耐性」しか持つことができなかった琉衣は、酷い扱いを受ける。ある日、妹の身代わりとして獣人がはびこる隣国皇帝の元へ降嫁させられることになった琉衣。
当の皇帝は猛毒を宿す“魔鳥”の血脈を引いており、自身も全身に猛毒を持つ「蠱毒（こどく）の帝王」と呼ばれる人物だった──。
孤独な王と、身代わり聖女の「政略結婚」。交わるはずのなかった二人の運命が、少しずつ動き出していく。