Found this weapon on my warlock in shattered halls, dropped off some random trash mob. Here's how it works:

Speeds casting up by about 22-23%%, 2.5 shadowbolts are 1.9. However, it's more of a proc cooldown, as in u will never get back to back, but if u haven't been fighting for a bit (between pulls), it will almost always proc off the FIRST or SECOND cast. That means a casted dot (not off the dot ticks, just the initial cast of the dot), or a spell, such as immolate or shadowbolt. This can actually be a nice thing, because it is garunteed to proc once a fight at least, and you can also predict when it's going to proc (its about a 20 second cooldown). That means you can begin life tapping or evocating or whatever, and you will often have it proc off the first cast.

the sword itself looks like argent avenger with demonslayer on it. Very good weapon, and if you're debating whether to use if the proc is good enough, it is indeed a very nice proc rate.