It's polygonal self shadowing. Probably aggravated by displacement.

If you're using a greyscale displacement map where middle grey is functioning as 0, run your displacement map ( gamma 1) into a math subtract node as shown below.

Also you can minimize polygonal self shadowing by lowering your shadow min bias on your light to 0.5 or lower, I've found 0.35 works well for me.

And finally, adding a level of subdivision is helpful for clearing up raytracing artifacts such as this.

---

W10 Pro, HP Envy X360 Laptop, Intel Core i7-10510U, NVIDIA GeForce MX250, Intel UHD, 16 GB DDR4-2400 SDRAM, 1 TB PCIe NVMe M.2 SSD

Mudbox 2022, Adobe PS CC, Poser Pro 11.3, Blender 2.9, Wings3D 2.2.5

My Freestuff and Gallery at ShareCG