I'll play around with it more later today and post back here. I literally had thrown the mesh, mat and hdri together and hit render...then saw this oddity.

The HDRI is called "mausoleum hallway" and is available on Keith Bruns' site; www.unparent.com, the hdri settings I used are posted there above.

The Terrain had been placed right in the middle of the scene.

I just tried replacing the the Terrain with just a sphere (with the same mat), and it just looks like a normal render.

I did have the Max Ray Depth and the TIR pumped up, so I put it back to its default settings, and the light casting looked the same. Must just be the shape of the Terrain making this effect.

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