The Kosst Amojan or Koss'moran was a Pah-wraith or group of Pah-wraiths.
It was thought to be a Bajoran legend that told of false Prophets who were cast out of the Bajoran celestial temple. The Bajoran word Koss'moran came from the verb kosst that meant "to be," and amoran, which meant "banished." (DS9: "The Assignment")
In 2374, the Kosst Amojan inhabited the body of Jake Sisko in order to bring forth the Reckoning. The possession was ended out after Kai Winn Adami flooded the station with chroniton radiation. (DS9: "The Reckoning")
Ambiguity exists as to whether the Kosst Amojan is an individual or another name for the Pah-wraiths. Kai Winn refers to it from an ancient prophecy as an individual in "The Reckoning"; however, in "Shadows and Symbols" the Sarah Prophet refers to the Pah-wraiths which were defeated in that episode as the Kosst Amojan. Furthermore, in the final episodes of the series, Kai Winn herself refers to the Pah-wraiths in the Fire Caves as the Kosst Amojan and it seems to imply that the Kosst Amojan is in fact the entity that possesses Dukat during his duel with Ben Sisko. It is possible that the individual Kosst Amojan (whose whereabouts remain unknown) entered the wormhole with the Pah-wraiths at the end of "Tears of the Prophets" and subsequently both were destroyed, or the prophecies as Winn knew them in "The Reckoning" were wrong and the Kosst Amojan is in fact the Prophets' name for the Pah-wraiths. Another possibility might be that it is a group of Pah-wraiths.
The group was originally named the "Koss'moran", and both that and "Kosst Amojan" mean "to be banished", but according to the Star Trek: Deep Space Nine Companion (pp. 389-390), René Echevarria altered the name (with Ira Steven Behr's approval) because he didn't like the "moron" part of the original name.