The Infernal Forge is a new weapon provided by the Cataclysm mod. Although it can also be used as a Pickaxe, and eventually upgraded, giving it a new special ability to be used in combat.
How to Get the Infernal Forge
There is no way to obtain the Infernal Forge through crafting. Currently, the only way to obtain it is by defeating the Netherite Monstrosity, which can be found within the Soul Blacksmith, which is inside the Nether dimension.

Currently, the Netherite Monstrosity is probably the easiest of all bosses to beat, which Cataclysm provides. Therefore it’s likely that the Infernal Forge is one of the first weapons you will receive.
Each time the Netherite Monstrosity is defeated, it will drop the Infernal Forge alongside a Monstrous Horn. These are used to upgrade the Netherite Helmet, turning it into a Monstrous Helmet and giving it some unique powers.
About the Infernal Forge
Even though the Infernal Forge is a weapon, it can also be used as a Pickaxe. It’s similar to a Diamond Pickaxe, so you can mine ores like Diamond and even break Obsidian blocks.
But where the Infernal Forge shines is as a weapon. As a melee weapon, it has an Attack Damage value of 13 and an Attack Speed value of 0.8, so it deals more than a Netherite Sword, without any Enchantments applied.

There is also a special attack, which can be done by right-clicking while holding the Infernal Forge. Doing so will cause damage in a small area to any nearby enemies, knocking them up and then causing them to take fall damage. This ability has a short cooldown.
Upgrading the Infernal Forge
One way to upgrade the Infernal Forge is by applying Enchantments. Your options include Sharpness, Smite, Bane of Arthropods, Knockback, Fire Aspect, Looting, Unbreaking, Silk Touch, Efficiency, and Fortune.
These can improve the Infernal Forge’s use in combat or as a Pickaxe. Though it can be further upgraded inside the Mechanical Infusion Anvil, it can be combined with a Void Core to turn it into the Void Forge.

Doing so will cause the Attack Damage and Attack Speed to remain the same. Though when right-clicking, it will now summon Void Runes in a fan shape, dealing additional damage to nearby targets. And it can still be used as a Pickaxe too.