Crushing is the hardest difficulty mode in all of the original games, Uncharted: Drake's Fortune, Uncharted 2: Among Thieves, Uncharted 3: Drake's Deception, Uncharted 4: A Thief's End, and Uncharted: Golden Abyss. It's unlocked in the first three games and Golden Abyss by completing the game on the Hard difficulty, without lowering the difficulty level, and is unlocked in Uncharted 4 by confirming twice on playing Crushing when selecting a difficulty for the story for the first time.
Most notable changes are that Nathan has extremely low health; simply trying to shoot can turn out to be fatal if the enemy presence is high enough. In turn, enemies have more health, so running out of ammunition is more likely. In addition, the player's first Crushing playthrough doesn't allow activating cheats or spawning weapons. (Cosmetic changes, such as skins, filters and mirror world are allowed, however)
In Among Thieves, Crushing can also be chosen as a multiplayer Co-op objective difficulty, while all co-op modes in Drake's Deception have the option to have the difficulty as Crushing. It can be unlocked by completing at least one Co-op objective mission on Hard mode, which itself is unlocked by completing at least one on Normal mode.
Main Story Rewards
Finishing either game on crushing awards the Charted! - Crushing medal, which is worth 150 medal points in Drake's Fortune and $40000 in Among Thieves. A trophy, also called Charted! - Crushing, is awarded in the three games along with the medal (update required in Drake's Fortune).
In A Thief's End, completing the game on crushing also awards the trophy Charted! - Crushing, only difference being is that there is no medal awarded for completing the game on crushing, given that there are no medals in A Thief's End.
Completing Crushing is the only way to unlock certain character skins and weapons on the Remastered versions of the three games.
Uncharted 2: Among Thieves Multiplayer Rewards
When any Co-op objective mission is completed on Crushing, the Masters of the Universe medal is awarded (worth $5000). If a Co-op objective mission is completed on Crushing with only two players, the Back to Back medal is awarded (also worth $5000).
Uncharted 3: Drake's Deception Cooperative
When playing Co-op Arena, enemies melee'd will almost always drop a grenade, armored soldier will almost always have a Kickback Booster, players will only have 10 lives overall, and finally all soldiers will be wearing clown masks (except for the Brute).
Uncharted 4: A Thief's End Changes
Due to the small gameplay changes in Uncharted 4: A Thief's End, the difficulty level has a few new limitations. During stealth, the detection meter is disabled, thus requiring the players to rely on their knowledge of the segment to move around. In addition, tagging enemies is also disabled, forcing players to keep track of their enemies. However, if an enemy produces noise, they will still briefly appear as a white outline through walls, as Nathan can still pinpoint their location by sound.
Notably, if the game has been completed at least once before, it's possible to use any unlocked perks and weapons that have been unlocked. Unlike the previous games, Uncharted 4 doesn't disable their use during the initial Crushing playthrough.