Every character has a dominating characteristic or rely on something to give them their identity. By taking away ones identity you have achieved three things: you send them hurling into their darkest hour, you make them reevaluate their values, and subvert readers’ expectation.
The darkest hour is when readers and characters begin to wonder if success is but an illusion.
To take away everything a hero holds dear, you must first destroy their main method of re-obtaining everything they hold dear, spiraling the character into a subjective endless hell. To many stories don’t do this, so when external circumstances push, they still have the ability to harden under pressure, assuring a dusky hour. Where’s the fun in pushing a hero in fearful circumstances? But destroying the hero’s confidence whether it be by killing a friend or destroying their magical sword, then pushing them under the bus, elevates writing into divinely screwed up, entertaining territories.
To take away ones identity is to force a character to crave a new identity.One of my favorite examples of this is when Jamie from ASOIAF lost his hand, transforming him into a irredeemable child murderer to Goldenhand the Just. The hero starts to rediscover life through a million different angles.
When a character pulls the same card again, the plot grows stale and predictable. Imagine if a character renowned by his willingness of never giving up defeats the enemy by never giving up. Now imagine, if a character renowned by his willingness of never giving up defeats the enemy by giving up and asking for help. If a hero uses the same trump card every game, take away that card and he’ll amaze you with a million different straights, dragons, and royal flushes.