A Bloodstained Envelope is a quest starting item dropped by event mobs during the Scourge Invasion event.
During the Scourge Invasion, non-elite mobs around the Necrotic Shards occasionally dropped one of 6 possible letters, of which this was one.
Each letter started a quest. Once accepted, the inventory item was replaced by a like-named item that was readable. This replacement item was a quest item for the associated quest.