What Aeternium Is
Aeternium is BetterEnd's top-tier material. It appears on the wiki as an Ingot, a Block, and a full set of tools, weapons, and armor, plus intermediate parts like the 
Aeternium Forged Plate, Hammer Head, Pickaxe Head, Sword Blade, and Spear Tip. You cannot simply mine it as an ore in most setups — you gather the raw components and assemble them.
Gathering the Pieces
Explore the deeper End biomes and any Aeternium-bearing structures. You will need Aeternium Ingots plus the specific forged parts for each item. The 
Aeternium Smith Hammer is the tool used at the 
Aeternium Anvil to shape ingots into heads, blades, plates, and handles. Craft the Smith Hammer first, then use the Anvil to produce the parts you need for the gear you want.
Crafting Order That Works
1. Make the Aeternium Smith Hammer.
2. Place an Aeternium Anvil.
3. Forge an 
Aeternium Sword Handle, then a Sword Blade, then combine into the 
Aeternium Sword.
4. Repeat the pattern for Pickaxe Head + handle, Axe Head + handle, Shovel Head + handle, Hoe Head + handle, and Spear Tip + handle.
5. For armor, forge Aeternium Forged Plates and combine them into Helmet, Chestplate, Leggings, and Boots.
Aeternium gear outclasses vanilla netherite in most BetterEnd setups, so it is the natural end goal of an End playthrough.