The Mechanical Fusion Anvil is one of the newer additions to Cataclysm. It’s similar to a regular Anvil and allows you to fuse items together, creating entirely new ones which you might prefer to use.
How to Craft the Mechanical Fusion Anvil
Crafting the Mechanical Fusion Anvil isn’t easy and not something you’ll be able to do without first beating the Harbinger. Currently, that’s the only way to acquire Witherite, which is required in the recipe for the Anvil.

After defeating the Harbinger, it will drop a Block of Witherite, which can be taken into a Crafting Table, turning it into 9 Witherite Ingots. Crafting the Mechanical Fusion Anvil requires 6 Witherite Ingots, 2 Redstone Blocks, and a regular Anvil.
Once crafted, the Mechanical Fusion Anvil can be placed in the game world, and it has some nicely animated textures. Then you can use it, allowing you to combine weapons and armor pieces to upgrade them.
How to Use the Mechanical Fusion Anvil
Mechanical Fusion Anvils can be used to upgrade gear by combining powerful items together. This might combine their abilities or give them new ones altogether. The process is free but requires you to unlock powerful items from Cataclysm or the Elytra.
An excellent way to see the uses for the Mechanical Fusion Anvil is to hover over it in your inventory and press the ‘U’ key while installing the Just Enough Items mod.

Placing a Gauntlet of Guard and Bulwark of the Flame into the Mechanical Fusion Anvil will create the Gauntlet of Bulwark. It’s a melee weapon that deals 11 damage. But holding right-click will knock enemies back and cause you to charge at them, dealing large amounts of damage.
The Infernal Forge allows players to deal damage in a small area, and the Void Core can summon damaging Void Runes. Merging these items in the Anvil creates the Void Forge, similar to the Infernal Forge, except it summons Void Runes in a fan shape on the right-click ability.

Ignitium chest plates can be combined with the Elytra. That way, you have the ability to fly while also having the defensive stats and abilities of the Ignitium Chestplate. This item is simply called the Ignitium Elytra Chestplate.
Finally, the Wither Assault Shoulder Weapon can also be combined with a Void Core, creating the Void Assault Shoulder Weapon. Instead of firing Wither Missiles, it fires Void Howitzer. These summon Void Runes in a large area, dealing massive damage to multiple mobs.