Ender Crystal

Ender Crystals are only seen in The End. They can only be destroyed by using TNT on it four times. If an Ender Crystal is destroyed in the End, your progress is saved on that level, meaning that you'll always start back at that spot when you lose. Until you go to the Level Select screen. The game ends if all of them are destroyed. Destroying these crystals can be a difficult task due to the TNT's expense.