True Sky is a procedural sky system for Blender. It can create realistic and versatile sky backdrops for your outdoor scenes featuring standard elements like a sun, atmosphere/fog, clouds, stars, moon, all the way to more extravagant features like auroras, planet rings and more. Aside from the sky itself, the addon even generates a terrain in the form of a full planet.
True Sky is primarily meant for use with the Cycles render engine. Eevee is not fully supported. However it is possible to convert the procedural sky into an HDRI and use that to light your Eevee renders.
One of the stand out features of True Sky is the generation of beautiful, realistic, procedural clouds.
While the addon is technically impressive, the detailed procedural visuals come at the cost of rendering time. Therefore it may be a good idea to only use it for scenes that require a detailed sky render. For less demanding shots it may be wiser to have the procedural sky converted to a standard HDRI.