Ancient Debris is the name of the main source of netherite scraps. How rare is it? According to some sources, it is extremely rare, the second rarest ore in the game called Minecraft, added in the Minecraft 1.16 update for Minecraft Java Edition and Minecraft Bedrock Edition.
It is found exclusively in the Nether between y levels 8 to 22. The ore floats on the lava in item form, cannot be burned, and has the same blast resistance as obsidian but is able to be moved by pistons.
Ancient debris is created in the Nether in the form of veins of one to three blocks. They are able to be obtained when mined with a diamond pickaxe or something better. Apart from that, it is also generated only with either blocks or lava surrounding all 6 sides, never open to air even though in some rare cases it generates under a twisting vine, making it visible from the surface.
You can gather ancient debris quickly in three ways. The first one is TNT explosions that can destroy the netherrack, which has a low blast resistance but leaves ancient debris. However, this method is quite expensive. Exploding beds also work well. It is cheaper compared to TNT explosions, but it is more dangerous as its blast range is higher and it sets fire to nearby blocks and players. The alternative way is to branch mine using a diamond pickaxe with Efficiency V. The weakness is it will break quickly, so it is better to have unbreaking and or mending on it. For your information, ancient debris is most commonly on y=15, but if using beds, it is better to go a block or two in order to avoid dealing with lava lakes.
Take a note that the Fortune enchantment does not work on the ancient debris, similar to iron and gold ore. Besides, it also does not drop any experience.
The easiest and the most efficient method to find the ancient debris is by digging a very long two-block high, one block wide tunnel at y=12 to y=15. Then, locate TNT blocks at 4 blocks intervals along with it. As explained before, the TNT is a bit expensive, but it is really satisfying to watch and exposes quite a ton of ancient debris.
The only official usage of the ancient debris is for smelting to make Netherite scraps, which you can use to make netherite ingots.
You need to get a diamond or netherite pickaxe if you want to harvest the ancient debris, otherwise, it will drop nothing. For your information, it will not generate in explored chunks of already generated worlds before the Minecraft 1.16 update.
Ancient debris is really one of a kind that is able to be found in the lowest layers of the Nether. The reason why it is so essential is that it is the most effective way of getting your hands on the Netherite scrap, which you will need to obtain Netherite. It is definitely an amazing rare material that can be used to help getting the most powerful gear in the game.