Players have discovered that a 2-Handed Epic Polearm drops at Item Level 593 from various locations in Azj-Kahet!
Since Early Access, players have reported getting some drops of an Item Level 593 2H Polearm Befouler's Syringe which drops as Warbound Until Equipped. This item can be upgraded all the way to Veteran 4/8 or Item Level 604.
Interestingly, this item has no main stat on it, making it useful for any class that can use a 2H Polearm including Death Knights, Feral and Guardian Druids, Survival Hunters, Brewmaster and Windwalker Monks, Retribution Paladin, DPS Warriors. This item level at 604 makes it the highest possible weapon in the game until Season 1 starts.
We don't know exactly how it drops and the best guess at the moment is it appears to be a World Drop from the Azj'Kahet zone. If you look at the comments on the Item Page, you'll notice that one user has reported as farming it from Elites in the Open World and another got one in a Follower dungeon. I've also received other reports of it being found while questing, so our best guess is doing things (dungeon included in Azj'Kahet.
If you got this weapon to drop, please leave a comment on where it dropped!