Gore Nests are portals from Hell to Mars. They have to be cleared to remove lockdowns of certain areas in the game. They are often lightly guarded but taking one out calls in more demons from Hell.
Studies of demons upon entering this dimension have shown that their conduct is not purely vindictive - there is a method behind their actions. When a demon captures their prey, the fresh kill is taken to a temporary ceremonial site where arcane rituals are performed on the pile of blood and gore.
When enough corpses have been gathered, the ceremonial site becomes a "Gore Nest". These sites, imbued with Hell energy from the rituals, act as umbilical cords to Hell. Extreme caution must be taken when approaching a Gore Nest. Attacking the nest, or indeed any demons within close proximity to the nest, will act as an alarm and siphon more demons from Hell.