Cannibal Magical: Niounomiya Siblings, Masters of Carnage

The No. 1 Killing Name, "Niounomiya", has finally arrived!

".....Specifically, what are you researching, Assistant Professor Kigamine?"
"——Researching not dying."
Invited to monitor the bizarre research on Kuchiha Madoka—a girl who lives eternal—Ii, the Nonsense User, visits the ruins of a clinic in northern Kyoto with antique apartment dwellers Ichihime Yukariki and Kasuga Kasugai.
However, what awaits them there is a terrible "fate" itself!
The Niounomiya siblings, the number one killers in the world, make their debut in this exciting new coming-of-age story.

(Source: Kodansha, translated)

Note: Includes two extra chapters.