"Ultimate Vault Hunter Mode is unlocked for a character once they have completed the main story missions in True Vault Hunter Mode and reached level 50. 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." -Gearbox
This mode also adds Legendary Loot Midgets which have a chance to drop Legendary loot, E-tech relics, and new Pearlescent loot. Pearlescent loot is only available to players 51 and above.
Bosses now have a higher chance to drop legendary loot as well as chests also have a higher chance to drop rarer loot.
Enemy health is increased 4x and also have minor health regeneration.
Slagged enemies are now slagged longer and take 2-3x more damage.