Halloween III: Season of the Witch (1982)

Kids all over America want Silver Shamrock masks for Halloween. Doctor Daniel Challis seeks to uncover a plot by Silver Shamrock owner Conal Cochran. An apparent murder-suicide in a hospital emergency room leads to an investigation by the on-call doctor, which reveals a plot by an insane toy maker to kill as many people as possible during Halloween through an ancient Celtic ritual involving a stolen boulder from Stonehenge and Halloween masks.