This looks like self-shadowing due to the terrain geometry, possibly from Ambient Occlusion.
Do you get these artifacts if you switch off raytracing? (try a small area render).

Are you using Ambient Occlusion on an IBL light? Or on any lights?
If so, I would suggest try increasing the Bias setting bit by bit and see if it makes a difference.

(edit) what about the rings of light? I don't see any.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)