env_cubemap is a internal point entity available in all Source games.

Declaring areas for cubemaps to cover is simple, just place an env_cubemap point entity inside the space of a map. When the map is compiled with VBSP, world geometry surfaces automatically associate themselves with the nearest env_cubemap and will use the cubemap generated from it. Entities associate themselves with the env_cubemap closest to their origin (alternatively, a cubemap can be applied to specific brush faces in the cubemap's properties); moving entities will dynamically change which cubemap they use. It is important to choose env_cubemap positions properly for both aesthetic and performance issues.

MapCore
Level Designer
CSGO Maps Workshop