You are started out with only 8 hearts and 3 lives. Everytime you die, you will lose 1 life, and once it reaches 0, you permanently become a spectator.
if you lose your last life in an unfair circumstance. Issue the command:
/scoreboard players set @p HardcoreLives 1
and go out of spectator mode

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There are 9 barriers which permanently lock the first 9 slots of your inventory, leaving a total of 18 slots in the inventory (Not counting the hotbar)

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There is also a structure void which permanently locks your head slot. This gives you only 8 hearts permanently, and locks your helmet slot permanently.

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Oh yeah, and you cannot regenerate naturally by having full hunger

You do not get any experience throughout the game, so the only way to get enchanted items is through villager trading

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Zombie Pigmen are ALWAYS hostile towards the player now

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There is a Mining XP bossbar which you need to level up to unlock different ores

0 Notches - Stone + Everything else
1 Notch - Coal
3 Notches - Iron
6 Notches - Gold + Redstone
10 Notches - Lapis Lazuli
16 Notches - Diamonds
20 Notches - Emeralds

A Notch is a term for each columns on the bossbar

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WHEN YOU EAT AN ENCHANTED GOLDEN APPLE, YOU WILL RECOVER 1 LIFE. HOWEVER, THE MAXIMUM IS ONLY 3 LIVES

WHEN YOU EAT A GOLDEN CARROT, YOU WILL RECOVER SOME HEALTH, BUT SINCE IT HAS OP SATURATION, YOU CAN EAT ROTTEN FLESH AFTERWARDS TO LOSE MORE HUNGER TO EAT MORE GOLDEN CARROTS

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After you beat either the wither or enderdragon, or both. You will get a trophy beside your lives to remind you of your success.

I brought back the old enchanted golden apple recipe. 1 Apple surrounded by 8 gold blocks.