The information in this article is from a fan game or fan fiction and is therefore not canon.
The Blow Gun was a makeshift weapon that Graham used against Llowh'wof when he was cornered in the swamplands. He inserted a silver needle (found in a haystack) into a hollow reed and then dunked the needle's point in poisoned water. He finally blew the poisoned needle at Llowh'wof, vanquishing him.