The Trinity Ghoul exotic bow is supposed to get an AOE electrified arrow on a precision kill, but there have been many a time in which you are not granted the arrow on a precision kill. I noticed that this occurs more often on skinny humanoid targets such as the Fallen and the Scorn; I rarely get this issue whenever I get precision kills on the Taken, however. EDIT: I would also like to point out that I noticed that this bow would sometimes get the arrow when I DON'T land a precision headshot. It's weird that way.