I only used it up to level three before I remade.
It fires a light-brown arrow that does normal damage, then leaves behind a green cloud that heavily damages anything that passes through it.
The cloud appears about half a second after impact, meaning it'll do piss-all if the target's moving. The hitbox for the cloud is slightly smaller than the cloud itself, which makes it irritating to see the cloud touching an enemy and...nothing. It's useful if you hit the first enemy in a line, or something. It's fine against bosses, but not great because it only hits a few times before it dissipates.
I don't know, maybe it suddenly becomes awesome at higher levels, but at level three I wasn't feeling it.