The Hyozan Reckoners were a group founded by Hidetsugu and Toshiro Umezawa. Its members were bound by a magical blood and kanji magic oath to avenge all deaths within their ranks, and suffering dire consequences if they betray other members.
The novels mention an oath that Toshi writes next to Kobo's body:
|“||We will kill you.
We will burn your fields, steal your treasure, destroy your house, and enslave your children. We will murder your spouse, poison your pets, and blasphemy on the graves of your ancestors. We will do all this, and the only way to avoid it is if we cannot find you.
We've already found you.