Migawari Seijo wa Moudoku Koutei to Saikou no Tsugai wo Mezashimasu!

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.