The Altar of Decay is a special crafting station located in the Brackenhide Hollow dungeon. When interacted with, it allows you to craft profession items that require “an altar infused with powerful decay magic”.
This isn’t available in the world, only in the dungeon instance of Brackenhide Hollow.
The Altar of Decay is there in Normal, Heroic and Mythic difficulty of the Brackenhide Hollow dungeon.
It is located in the final boss room of the Brackenhide Hollow dungeon. It is accessible once the final boss of the dungeon is defeated by running up one one of the ramps to the side of the final boss.
Although I now realise that it strongly hints where to find the location of the Altar in the text underneath the recipe, I found myself searching for its location. I hope this helps and that you’re enjoying Dragonflight so far!