Made by placing 9 cobble (for the overworld variety) in the clock-face pattern shown in the necronomicon. The altar is activated when the center stone is shift-right-clicked with the necronomicon.
Altars can be constructed in the other dimensions, but they require their own local materials. Corallium cobblestone brick for the Abyssal wastes, etc.
Used for the rituals described in the Necronomicon.