I already wrote about the Altar of Chaos a few years back.
There are a few altars around in various underground structures or cave complexes. All of them share this strange mix of impossible geometry and nonexistent colours that make you at least dizzy, sometimes positively sick, just to look at. All of them react when interacted with, not all of them in the same way but they all remain mostly unpredictable.
At least one of them has been broken into fist-sized pieces. Looking at the pieces will probably still give you a good headache, but they don’t seem to have any other detrimental effect.
The altars often seem to call strongly magical beings that make their lairs nearby as if by chance. Twisted faeries, hags, powerful witches or warlocks or demons.
Few people know of the altars, and none of these know who made them; the only thing known for sure about their origins is that they are ancient. Their creators may be one of the mythical elder races that were long gone when the dragons came into the world.