the item that undermines the Obstacle Character's efforts
The Obstacle Character's Critical Flaw undermines his effectiveness against the Main Character in general, but especially in regards to his Unique Ability. The Obstacle Character in any story has a Unique Ability which makes him uniquely qualified to thwart the Main Character. But in his character as well is a Critical Flaw which prevents him from just totally overwhelming the Main Character. This is again a trait which is unique to this particular character.