Slaughter Demon was a combat knife with a fur guard and a black handle. It has a wide-edged blade with two holes and bandages at its base.
Slaughter Demon was a dagger imbued with cursed energy that even non-sorcerers could use to exorcise curses. Satoru Gojo gave it to Yuji as a way for the young man to fend off curses on his first field test.
Coupled with his incredible physical prowess, Yuji used Slaughter Demon to make short work of a cursed spirit in close combat. It also helped him cut off the arm of another curse to free a hostage during the same test.
Satoru had originally planned on teaching Yuji to control his cursed energy with Slaughter Demon only being used for emergencies. However, while Satoru was away on a mission Yuji and his peers were sent to the Eishu Detention Center. There, a Finger Bearer attacked Yuji and broke Slaughter Demon.
At the 30th Annual Kyoto Goodwill Event where Yuji returned to school, Maki asked for Slaughter Demon back. This made Yuji realize that it originally belonged to her before Satoru gave it to him. Rather than admit that he broke Maki's cursed tool, Yuji claimed Satoru still had it.