Nice sword. There is a quest to steal something in Verdistis which gives you an agility boost (in a game started in the latest version of DD, you can complete the quest and buy back the item), so even without new charms or equipment bonuses you could be able to equip Brightblade after your next level up.
There is one potential problem, though. Mixing any two of Frost on a weapon, Stun on a weapon and the passive Stun skill can sometimes result in creatures hanging in mid death animation, and not dropping items or giving experience. An area effect spell that doesn't need a target, such as Hell Spikes, can force the animation to end, though. Some people also reported that waiting a bit or sleeping would also work.
I think
DAD's item editor could remove the Stun bonus, if you wanted.
If you cast an area effect spell that doesn't need a target (such as Poison Cloud) near, but not directly on an NPC, you can do damage and kill them without turning them hostile or loosing any reputation. Most or all essential NPCs are protected from dying in the latest game version.
In the case of Seth, you can even use the
Reviving dead NPCs procedure to kill him repeatedly and get multiple copies of the sword.
