Prerequisite: BAB +1.
Benefit: If at least one square adjacent to you is impassible to a foe (a wall, a pit, or some other terrain that your foe cannot move into), that foe cannot flank you. Additionally, as a move action, you can harass any foes within your threatened area. Enemies must treat the spaces you threaten as difficult terrain, spending double the normal movement to pass through the area.
Adventurer’s Handbook: Genius Guide Volume 1