I haven't seen this weapon, but once I made my own "powerful" sword that was based on Warhammer 40,000's Daemon weapon "Hellblade" (Carried by Blood Letters).
The weapon did 30 fire damage and ignored 150 points of armour (hoping it would ignore all armour encountered and even the fire resistances from the Resist Fire spell. Well, for levels 1-6, that is). However, because it was a fire typed attack, it would decimate buildings with incredible ease on top of it's intended anti-unit purpose.
The weapon was soon promoted to an artifact due to its sheer power but also, I added a negative trait to it, one that suited the weapon's "background" (W40K lore here, but I wont go into it). The trait it self was -50 hero mana, meaning to hold the weapon was to loose access to the higher level spells. Of course, high level heroes could bypass that with the sheer mana amount they could get, but then again, what mage would want this weapon? It was made for warriors and the effects prevented such from being able to use spells along side the weapon.
I chose to do this instead of a simple negative spell-casting trait because of how over used that is for a negative effect. I also did other things to other weapons like negative mana regenerations (-100% should stop mana regen altogether, perhaps that's another thing I may try on another War God forged weapon?) and a constant resource outcome, which meant while the item was equipped, you would loose 1 point of crystal per second (done through a negative resource income).
This was my power line for the Hellblade's negative mana.
<Power id="2" type="hero skill" data="5" level="-5">-50 Mana</Power>
That's hero skill 5 (which is "Lore" at -5 points, making -50 mana points altogether), but notice the power bit at the end - the negative hero skill didn't turn out well when the game came to adding in-game text, so I had to over write that with that power bit.
Anyway, that's just an idea with an example.