While running like a crazy person around (I tend to do that) I walk upon this place (sorry for low quality... phone pictures) in south east, near the Nilfgaardian border:
It looks like a semicircle of 5 altars of some sort, each altar having a symbol on him (swallow, sea monster, dog, mare, and another bird). Looks important/fun but is nothing around (they don't glow on witcher senses, can't interact with them).
From what I remember my first playthrough didn't get me here (but that was like 2+ years ago so I may be wrong) so I don't think is related to the main quest.
So... what is this place? Anything here? My guess is that is not just for flavor but...