If it rebounds just the next hit of the opponent after you cast it, then how can it be so useful? I mean, you can't know how hard the opponent is going to hit.

 

How can I use it in pking? Just use it every 30 seconds and hope the opponent hits hard?

Well exactly, some opponenets will switch to like rune knives once they see it cast. But lets say you are flighting somebody with a maul and they get hit by veng, then you have a chance to spec and kill them.

 

It just becomes another tool to increase your KO potential in pvp.

 

So yes, just cast it every thirty seconds and hope your opponent hits hard. Also be ready to follow up with a spec.