He's being chased by the demon lord, then they somewhat agreed to go for this "trials" because the demon lord needed his death spell for trial 4 iirc, they travel along with all the other people, then enter the first trial and went separate ways on the second one, he completed the second one...
iirc on the novel a person found a dead body on his neighbor house, and then saw him walk away or something like that, then that lead them to conclude that he was indeed a rat