How does Shadow Instanct work?

I heard it only gives 20 Atk.buff when u have 5 KP points?

It gives 50.

It gives 50.

Kinda hijacking for confirmation.

It stacks with Karma, right? Because it would be stupid for it not to.

And does Karma stack with Rage? If so, can all three of them stack with one another?

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.

Karma does not stack with anything.

Karma does not stack with anything.

Does it get cancelled out by lower attack buffs, or does it stay active unless you use/activate something that gives more than 30 ATT?

It will get cancelled. It's like a Warrior Pill etc.

Karma and Shadower Instinct stack.
It also stacks with Bless/advanced bless.