If I had understood it well, you were confuzed about why on NPCs it just adds 45% and on mobs it adds 90%?

Well, I don't really know the answer since I never made any test just because we already knew this, but what I guess is that maybe there are different formulas for each target, NPC got their own so do mobs.

And afaik NPC = Player, so on players I suppose it should add 45%.

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Sure, a lot of people gets confuzes of that.

This is what Focus Death erally does:

Aprox 45 ~ 53% more damage from skills and 90% damage more from normal attacks.

Just don't forget that all these bonuses is when you are facing them at their back.

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