It gives 300 weapon damage and 300 spell damage. The buff lasts for 20 seconds with a cooldown of 15 seconds meaning you could theoretically have 100% uptime on it, but you wouldn't necessarily have 100% uptime. I also don't believe this would stack in a group if multiple people were running it, not that it really synergizes well with group play.
A fairly mediocre nightblade that occasionally kills people