This is a simple server side mod, which changes your vanilla cobble/stone gen, to produce the deepslate versions when you build it below a particular Y level (default 15, configurable)
Feel free to use this in any and all modpacks
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Updated to be a couple of CraftTweaker scripts now, simply grab these scripts and put them in your scripts folder
Soon to be at https://legacy.curseforge.com/minecraft/customization/deepslate-generator-craft-tweaker-scripts
No files available for download.