The Ultimate Vault Hunter Mode (UVHM) is a new playthrough mode introduced in the Ultimate Vault Hunter Pack DLC
It is free, regardless of whether or not you buy the DLC, though it is better to have the DLC because otherwise you will not be able to level past 50
You to have to be level 50 to start it and must have beaten TVHM. Its an entirely new play through(3rd playthrough), which means the difficulty will be scaled much higher.
Ultimate Vault Hunter Mode is unlocked for a character once they have completed the main story missions in True Vault Hunter Mode.
Unlike other playthroughs, Ultimate Vault Hunter Mode can be replayed multiple times with players able to reset their overall mission progress at any time from the Main Menu.
No more tutorial missions -- characters in Ultimate Vault Hunter Mode begin in Southern Shelf with the "Cleaning Up the Berg" mission.
While playing in Ultimate Vault Hunter Mode, enemies and bosses will scale according to the player's current level or, if playing with others, the highest-level player in that party.
Gameplay changes in UVHM
Enemy health generally increased 4x.
Enemies now have a moderate amount of health regeneration.
Increased duration of slag damage multiplier effect.
Upped the damage that slagged enemies take from 2x to 3x.
Weapon swap speed increased to better facilitate slag use.
Enemies now more likely to drop ammo.
Loot Midgets are now "Legendary Loot Midgets" that can drop Legendary and other top-tier gear.