Devourers are strange black and purple creatures found in the Pre-Light Forest, Dark Forest, and occasionally on the border of the Olux Swamp. There are also "Escaped Devourers" that can be found in a cave. They are notable for their high health of around 8000.
Devourers(?) and Escaped Devourers can drop "Dark Soul In A Bottle"
These can be traded to a junk merchant in between Bucie and the river for 1 Emerald.