Just released the modular cliff system. An 18 piece collection of large rock formations to easily design cliffs, caves or whatever your level requires. 5 square shaped, 5 oval shaped and 8 flat rocks. All have 360° of usability. Each piece also has 3 materials: gray, desert and brown. As well as a 3 level LOD system. You can find them here in the asset store.
https://www.assetstore.unity3d.com/en/#!/content/41362