Penance Malediction was a form of hex wherein a victim was punished by his or her own subconscious by turning it against himself or herself.
Seeking vengeance, and, as later revealed, under orders from Warren, Amy Madison performed this spell on Willow Rosenberg, who continued to carry feelings of guilt from her murder of Warren. Consequently, Willow transformed into Warren shortly after kissing Kennedy. Over time, the hex caused Willow to be more like Warren not just physically, but also verbally, emotionally and mentally. Willow as Warren began to harm people around her casually and spoke and acted like Warren.
When Amy inadverdently indicated she was aware of what Kennedy was, the witch revealed that she was responsible for Willow's transformation through a hex, which she described as a "standard Penance Malediction".
The hex eventually broke when Kennedy helped Willow move on from her feelings of guilt, including Tara's death.