http://wikimoon.org/index.php?title=Dark_Mercury Dark Mercury, who has also been referred to by the unofficial nicknames of Darkury or Dark Sailor Mercury outside of the series, was the evil form of Sailor Mercury who appeared only in Pretty Guardian Sailor Moon. She first appeared in Act 21, after Sailor Mercury was kidnapped and brainwashed by Kunzite.
At first, she appeared like a normal, but cooler Ami (dubbed "Akumi" - meaning "evil beauty" in Japanese kanji characters - by the series production staff). But shortly thereafter she transformed into Dark Mercury, much to the horror of the other Sailor Senshi.
She was often disrespectful toward Kunzite and the other Shitennou, and even to Queen Beryl. She could, however, show kindness, such as when she repaired Nephrite's cape. (After this incident, Nephrite acted strangely in front of her, a possible hint that he might have had some feelings for Ami.)
Dark Mercury returned to normal in Act 28, after realizing that she had hurt Sailor Moon. After that, Ami had no memories of what she did as Dark Mercury.