Wanna add a RPG flavour to your Minecraft game ?
This command is for you !

And yes, this is only one command, you just have to select it and paste it in a command block and power it with redstone!
With this command, you can completely change your experience of the game.
I highly recommend you to use this on a new world as you can't access to everything if you don't have the appropriated level (especially Nether and diamond tools)

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* What does this command do ?
When the box is active, some mechanics are slightly modified..
New Life Regeneration System: You can't regenerate your life when you're fighting. When you're not fighting, your life will be instantly regenerated (60 seconds after the last hit you took)
New Leveling system: Bake cake, kill monsters, trade with villager and more to get experience and levels. With each level, you also get a little bonus
Mob Scalling System: Mobs are scaled on your level: the higher you are, the harder mobs will be.

* 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%

See the command

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This command has been made for Minecraft 1.8. It does work on Single player and Multiplayer
If you're experiencing heavy lag when using it, try to remove the box and replace it again in another place (few blocks away should be fine)