The Pah-wraith called the Evil One in Bajoran scripture, which possesses Jake Sisko to fight a Prophet on the Promenade and whose forbidden book Kai Winn later uses to free its kind.
Kosst Amojan is the name Bajoran prophecy gives to the chief of the Pah-wraiths, the outcast enemies of the Prophets, and is usually rendered as the Evil One.
It appears in the Deep Space Nine episode The Reckoning. An ancient tablet from the ruins of B'hala releases two entities aboard the station: a Prophet, which takes Kira Nerys as its vessel, and Kosst Amojan, which enters Jake Sisko. They begin a duel that prophecy says will usher in a golden age for Bajor if the Prophet prevails. Sisko lets it proceed despite the danger to his son, but Kai Winn floods the Promenade with chroniton radiation and drives both beings out before either can win.
The name returns in the final episodes, where Winn and Dukat read from the Book of the Kosst Amojan in the Fire Caves in What You Leave Behind.