How does Shadow Instanct work?

My understanding of it was that it gives 20 reguardless of how many KP you have but (at max) adds 6 additional attack per KP orb consumed (+30 additional) making the total +50 attack when all 5 orbs are consumed - lasting 60 seconds at max. As far as I can tell from videos- it stacks w/ Karma's buff. I do not know If karma stacks on other buffs like potions or rage.