Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Horizon[ON] Free.
Horizon[ON] v1.4.1
Horizon[ON] v1.0
Try the demo and see how Horizon[ON] looks out of the box!
Horizon[ON] is an all-round solution to make your horizons look great, even on small terrains. Terrains usually have to be much bigger than the accessible area, just to hide the square shape of your world. Horizon[ON] will extend your terrain as far as the eye can see.
With the latest update the visual fidelity has been upped to a level that is on par with, or even surpasses, current AAA productions. Your performance most probably even gets better by using Horizon[ON], since you don’t need huge and expensive terrains just to fill your backgrounds.
Works with unity terrains or mesh terrains.
Horizon[ON] is used extensively in the featured “Made with Unity” game D.R.O.N.E.
Notes:
Horizon[ON] 1.4 needs Unity 5.6.3p4
Horizon[ON] 1.2 needs Unity 5.2.1
For upgrading to a newer version please uninstall the old version first!