[Snake]

Oops, I did a nice overlook. Didn't knew Fade Out adds Sneaking buff to the Umbral.

Anyway, it costs 3M to use Fade Out, so it would be as fair as a gunner using Charged Shot followed by a basic hit. But since Umbrals are a thing, I can't ask for higher numbers.

Can it be increased to 25%? Doesn't seem to be an OP damage buff for me. Plus, Umbrals are not really a common race around the world (read: players), like Vampires, Kaelensian or Mechanations.
Having 'Sneak Attack' a special gimmick for Umbrals would be as fair as Leporidae's 'danger sense' passive being used with Kensei's Sheath Sword/Quick Draw. Or Corbie/Hyattr's Gain Air being used with Winged Serpent.