func_viscluster is an internal brush entity available in all Orange Box games. It allows you to manually tell VVIS that the leaves inside its volume can all see each other. This is useful in large open areas where visibility calculations would otherwise take a very, very long time to complete. It should be textured with
Warning: The brush should not cross the surface of a water material. If it really does need to be used over water, bring the brush level with the surface of the water on one side, and then create a second, separate func_viscluster on the other side. Never allow a func_viscluster brush to cross a water plane, or very strange things happen.