To make him unable to proceed fast? Maybe. And I think he is not in an illusion or why did his magic detection show him his people in a cell and not around him? Of course it could be done with another illusion magic, but that lunatic assassin really wants to kill both Will and his father so it would not make sense to only put him in an illusion. It would be easier to make it so that he will have his hands tied choosing between his father and underlings. I am pretty sure the head of guards and the others does not escapes with the dragon because the dad is already captured and held hostage so they can not escape or he will be killed.Σ(  ̄□ ̄||)

Im pretty sure they didnt actually kidnap them...