You have studied the techniques of fighting underground, and you can protect yourself from the dangers of multiple attackers whenever you can put your back to a solid wall.
Prerequisites: Shield Proficiency, Tunnel Fighting.
Benefit: If you have a ready shield with which you are proficient and one side of your space is entirely in contact with a solid wall, you cannot be flanked.
Special: A fighter may select Stoneback as one of his fighter bonus feats.
Source: Races of Stone, page 144.