Seraph (light weapon)
Baleful (dark weapon)
During Act 7, you can choose between Light and Dark weapon, and each is depending on your own class.
IMPORTANT NOTE:
I can never stress this enough. Make sure your weapon is 6 slotted before upgrading it! Meaning you must be able to put 6 gems into your weapon!
Its okay to use it as a temporary but once you start upgrading, there is no going back. Unless you are willing to start from Stage 1 again
You can purchase your weapon again and again at grand harvest square with Naryu Silver after you did Act 7 Chapter 1.
I find Celestial Basin a good way to farm for Naryu Silver if you need it. Just kill the field mini boss and they will drop 2 each time.
Next I will talk about which Class should go which weapon. Technically, all classed use baleful with some certain exception
Classes that goes baleful: Kfm, Bm, Destroyer, Sin, FM, GS and SF
Classes that goes Seraph: Warlock
Classes that might work both way: Summoner and Blade Dancer?
Summoners:
Summoners is a class that is pretty flexible.
They can either go Baleful or Seraph.
Personally, I would strongly recommend Seraph. Reason being, Your "F", Flying Nettles is one of your strongest skill. With Seraph, your Blair Patch(Flying Nettle) cooldown is reduce by 50%, which
means that skill dps is 2times more now! Also, it combos very well with your badge and MSP soul shield set, which further makes this skill stronger!
Again, its up to personal preference and its arguable that Summoners can go both way.
Blade Dancer:
To be honest, I don't play blade dancer. I only heard that some says seraph, some says baleful. So you gonna research this yourself.