It's not new to me since I watched the anime but damn that's still so sad for Urokodaki. He kept sending kids he trained for months or years to take the test but none never came back and he had no way to know what happened to them. Like the fact that it's specifically because he was there teacher that they were targeted and that it's because of the masks he gave them to drive bad luck away that the oni knew they were his students. Quite ironic in a sense I guess.