A sacred circle is dedicated to a specific deity and infused with divine energy.
Effect: A creature that has the same alignment as the deity a sacred circle is dedicated to gains a +2 bonus to attack rolls while standing in that circle. Most sacred circles fill an area 3 squares by 3 squares.
Usage: Use a circle dedicated to an evil deity with the aligned monsters (evil cultists) arrayed within the circle, improving their attacks. Or use a circle dedicated to a good or unaligned deity that the characters can use to improve their attacks against enemies with high defenses. Each option requires different strategies for the characters to use, and makes forced movement effects more valuable for one side or the other.
Published in Dungeon Master's Guide, page(s) 68.