The Soul Blacksmith is one of two dungeons that can be found in the Nether and is provided by the Cataclysm mod. It contains the Netherite Monstrosity, which is recommended to attack before the Ignis.
How to find the Soul Blacksmith
Two Cataclysm dungeons, the Soul Blacksmith and the Burning Arena, are spawning within the Nether. Both are exposed, so there is a good chance you’ll naturally come across the Soul Blacksmith, especially since it is a large structure.
However, players can also craft the Eye of Monstrous. It works similar to the Eye of Ender. When released, it will point to the nearest Soul Blacksmith, and only one is required, as there isn’t a chance for it to break.

To craft the Eye of Monstrous, surround an Eye of Ender with 4 Buckets of Lava, 2 Netherite Scrap, and 2 Blackstone. These won’t burn in lava and will float above it. So, if they do happen to fall into lava, they shouldn’t be too difficult to retrieve.
Alternatively, you can enter some commands, provided you have cheats enabled. You can use the commands /locate cataclysm:soul_black_smith or /locate structure cataclysm:soul_black_smith if you’re on Minecraft 1.19+.
About the Soul Blacksmith
The Soul Blacksmith’s layout is entirely exterior. As the structure is a blacksmith, it contains several valuable blocks that can be collected and returned to the Overworld or your base.
There are cauldrons, blast furnaces, anvils, and smithing tables around the blacksmith. A block of Ancient Debris is visible against the Netherite Monstrosity, the boss that resides here.

There is only one new mob that can be found within the Soul Blacksmith, and that’s the Netherite Monstrosity. It’s a tanky boss with 500 Health and 10 Armor points. The Netherite Monstrosity rests in the Soul Fire, awakening and attacking once you get close.
When fighting the Netherite Monstrosity, you’ll want plenty of Fire Resistance. During the fight, the Monstrosity will launch balls of lava at the player, dealing high amounts of damage. Though these are quite easy to dodge.

A good strategy for fighting the Netherite Monstrosity is to try and brute force it. Get close with an item like a Netherite Axe and deal as much damage as possible. That way, the Monstrosity will mostly use its devastating melee attacks, knocking you in the air.
However, it should be easy to deal with, provided you have enough health regeneration. Once defeated, the Netherite Monstrosity will drop the Monstrous Horn, used to craft the Monstrous Helm. It also drops the Infernal Forge, a melee weapon.
Secret Room
A secret room does exist within the Soul Blacksmith. Where the Netherite Monstrosity spawns in the Soul Fire area, you will find a Polished Blackstone Button on the left-hand side. It’s in the corner, facing upwards.
Pressing this opens a doorway in the back-right corner of the Soul Fire area, on the opposite side. However, you can also mine through the Cracked Polished Blackstone Bricks with a pickaxe.

Behind this doorway is a small room containing some valuable materials. There are Blocks of Gold, Ancient Debrit, Gilded Blackstone, and some Nether Gold Ore.