Ok, this thing is just crazy. It has some passive effects, but those are unimportant. You're paying the 13,000 gold for it's ability. It's ability does the following.
+300 weapon damage for 10 seconds.
Yep. It's quite crazy. Let suppose you have 1.0 attack speed. This means 3000 normal damage + your normal weapon damage. Someone with high attack speed (say, 0.5), is going to get 6000 normal damage out of this! 6000!
And here's an even better bonus... It applies to nearby allies. 3 people with +300 attack for 10 seconds = someone's in for a lot of pain.
If you're a DPS build, this is a must late-game item.