This command add a new leveling system with bonuses each time you get a level. It also add a new life regeneration system : you'll only recover your life if you don't lose HP during 60 seconds (delay reduced after level 3 and 9). But as your level goes up, monster will be tougher and tougher.
You gain experience by killing monsters, crafting some items or doing specific actions.
List of upgrades par level
Level 1 : Ability to use Iron tools and swords
Level 2 : HP Increased
Level 3 : Faster life regeneration, Ability to enter the Nether
Level 4 : Ability to use Diamond tools and swords
Level 5 to 8: HP Increased
Level 5 : Ability to enter the End
Level 9 : Faster life regeneration
Level 10 : All damage reduced by 20%
Compatible with multiplayer
WARNING : This use a lot of command block, when the box is created, make sure it isn't in only one chunk (if so, your computer will start lagging)
When the box is created, it require a lot of space above too.
The beast in action
Update: Levels are now a bit easier to get.