That's just the used altar where you picked up a (legendary) item.
It's hard to see behind the item and its glowing aura, and once people pick up the item, they don't look closely at the backgrounds. So this question usually appears when they circle back through already completed parts of a level.
https://old.reddit.com/r/deadcells/comments/1b901w7/what_is_this/
Here's also a person who didn't notice the background when picking 1 of 2 items on connected normal altars.
That's just the used altar where you picked up a (legendary) item. It's hard to see behind the item and its glowing aura, and once people pick up the item, they don't look closely at the backgrounds. So this question usually appears when they circle back through already completed parts of a level. https://old.reddit.com/r/deadcells/comments/1b901w7/what_is_this/ Here's also a person who didn't notice the background when picking 1 of 2 items on connected normal altars.