Ultima?

I am a peculiar kind of justice
Although some might see me as nothing more than Revenge
Old Testament, even; fairness in punishment,
And just as contemptuous of magic.

Justice? maybe Cloud and the party fight for the justice of their planet.
Ruby counterattack-ULTIMA
it's kind of old in Corel and good for attaking
It's a magic

I am a peculiar kind of justice
Fit only for commons, unworthy of a King
The kind applied roughly in dark alleys
With malicious intent, and more force than before.

See above
people of Corel use it, they're commoner and maybe Rufus is the king
Dark Alley maybe represent Mt. Corel
Can't really think of last line.

If it not ULTIMA, maybe Vincent?