Black River is a Unique Scythe. It can only be used by Necromancers.
Effect

How to get Black River
You can farm this unique from and Duriel, Maggot King
. You can also use our handy Target Farming Tool to see which dungeon is best to farm this unique.Andariel, Maiden of Anguish
"The scrolls describe a river of tar that separated the living from the land of the dead. Any unfortunate soul who tried to cross would be dragged down into oblivion." - Notes of Scholar Kamien