Kotharel the Harvester

Dreadhold was built to contain the most dangerous criminals on Khorvaire—mortal or otherwise. Kotharel is a radiant idol, a mighty angel banished from the plane of Syrania and condemned to walk Eberron for eternity. Kotharel claims to be a lord of death, the right hand of the dark god known as the Keeper.
  Exiled to Eberron in 710 YK, Kotharel soon established a cult of murderers who “harvested” innocent lives in its name. Kotharel’s cult survived for 23 years before the knights of Dol Arrah finally found the renegade angel. The battle that followed shook the foundations of Metrol, until at last the knights brought down the radiant idol. But death would not take the Harvester. After all efforts to destroy Kotharel came to naught, those who defeated it brought the fallen angel to Dreadhold and imprisoned it in a dead cell.
  Kotharel provides characters with a chance to talk to a powerful outsider under controlled circumstances. Should it be released, it could become a deadly enemy or powerful ally — although any alliance with Kotharel likely ends in betrayal.
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