It’s a bit obtuse, but creative
World of Warcraft: Dragonflight revels in creating tons of sidequest opportunities, but for crafters, you may want to make the Altar of Decay a priority. It’s used in crafting tasks that “require an Altar of Decay,” or “an altar infused with powerful decay magic.”
Here’s where to find that altar.
Head for, or queue into, the Brackenhide Hollow dungeon
The Altar of Decay isn’t a specific place on the world map itself: rather, it’s inside of an instance (Brackenhide Hollow), which makes it harder to get to. Brakenhide Hollow is a dungeon found in the southwest corner of The Azure Span, in the Dragon Isles. You can just queue into the dungeon directly, but the level range is 66-70.
If you’re the adventuring type, you can head there manually. The entrance is outdoors, right on a dock, with Tuskarr NPCs around it (coordinates 11.5, 48). There is an outpost nearby, which can provide a flightpath right near the dungeon. Note that you’ll need to complete it, so you’ll want to go in with a party (normal, heroic, or mythic will do).
Finish the dungeon, and use the altar at the final boss
After making it all the way to the end of Brackenhide Hollow, you’ll find an altar to use on those recipes. That’s it! Repeat the process if you want to use it again.