Normally, if you die, all of your possessions scatter on the ground.
If you have some sort of gravestone mod, your possessions are stored in a gravestone block, preventing them from despawning. Some gravestones are even designed to put all of your equipment back into it's original slots. This of course maks the game easier.
What if there were something like the opposite of a gravestone, to make the game harder?
Suppose that an undead mob is spawned in the player's place, and is given all of the player's equipment. He's persistent, and won't despawn, but obviously you have to kill him to get your stuff back.
And of course, he'll be wearing all of the armor you had when you died, and wielding your weapons... making him harder to kill.
And perhaps he'll even look a bit like you -- his skin is a modified version of yours, but converted to shades of green, and with light and dark reversed.