Koukyuu no Dokuka

A "poison consort" with access to all kinds of poison and a boy who is a "cross-dressing consort" solve the mystery lurking in the harem!

The time is the Great Tang Dynasty. A boy named Gyokuran, who entered the harem as a cross-dressing consort in place of his missing sister, meets the poison consort Dudu, who is said to have access to poisons from all over the world, past and present. The two of them must try to solve the mystery of the mysterious incidents that have been unfolding in the harem...

(Source: Kadokawa, translated)