Fabric Reinforced Obsidian
You might be looking for the Modern Industrialization addon version of this mod. You can find it here: Fabric Reinforced Obsidian (MI Addon)
This mod adds a block called reinforced obsidian, that can't be broken by the Wither. It also adds reinforced bars used to create the reinforced obsidian. The block itself comes from EnderIO and although the textures are similar, I recreated them in paint.net myself. This also means that they are subject to change.
Reinforced obsidian blocks allow the player to build a safe Wither cage with as little as 6 of them! (You could potentially just use the bedrock on the nether roof as a substitute for the block below the Wither. Anywhere else, though, that block is needed)
How to use
To make reinforced iron bars, substitute the iron ingot on the center slot of a vanilla iron bar recipe for a netherite ingot.
You can craft the reinforced obsidian with four iron ingots, four reinforced iron bars and one obsidian:
From version 1.18.1-1.0.5 forward, both reinforced glass and reinforced tinted glass have been added to the game. Just like their reinforced obsidian counterpart (and its variants), both blocks are wither resistant as well!
The reinforced obsidian glass can be crafted with four obsidian, four reinforced iron bars and one glass
The reinforced tinted glass can be crafted with one reinforced glass and four amethyst shards.
F.A.Q.:
Q: Will you port this to Forge?
A: No. If you want to do it, please contact me.
Q. Can you port this to [older version]?
A: No.
Q: I don't like the recipes! How do I change them?
A: Use datapacks.
Q: Can I translate this mod and provide the lang file?
A: Of course! Feel free to do so.