func_viscluster is a internal brush entity available in all Source games since Half-Life 2: Episode Two. It allows you to manually tell VVIS that the visleafs inside its volume can all see each other and should be treated as one. This is useful in large open areas where visibility calculations would otherwise take a very, very long time to complete. Poor use may be expensive for in-game performance. It should be textured with
mat_leafvisshow the leafs to still be intact.
func_visclusteron the other side. Never allow a
func_visclusterbrush to cross a water plane, or very strange things happen.