Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Terrain Tile Editor Free.
Terrain Tile Editor v1.2.1
Triangle Studios’ Terrain Tile Editor package provides an easy way to use tile sets in combination with Unity terrains. Simply paint tiles onto the Unity Terrain, and maintain all the flexibility you normally would when designing a map.
Terrain Tile Editor will enable you to layer up to 8 individual tile maps. Tile sets are created using the Warcraft 3 standard. This means that lots of materials have already been created by the community and can be imported in your project right away!