Weak Constitution Vs Mage Killer

First of all Spell damage reduction does not stack the highest one you have is the one that takes effect.(correct me if im wrong here Glorn)

Heroes start with an innate 20% spell damage reduction

I'm going to cover a number of skills here but we are mainly going to focus on Magekiller aura

Magekiller Aura Increases spell damage reduction on allied units by a set value in an area of effect.

Weak Constitution Sets spell damage reduction to a set value in an area of effect on non allied units.

Banish decreases spell damage reduction and slows a single target by a set of values.

The following items give spell damage reduction
Arch magus Cloak 25%
Gloves of Death 50%
Cloak of the Hidden 50%
Cloak of the Unseen 50%
Black knights mail 75%
Immortals boots 95%
Wooden Shield: 20% magic damage reduction (same as base reduction but I could be wrong)
Standard Buckler: 40%
Iron Kite Shield:60%
Prismatic Shield: 80%
Fiery Defender: 50%
Icy Barrier:50%
Unstable Arm guard: 50%
Decaying Shield: 50%
Stone Wall: 60%
Demonic Defender:60%
Reflective Icy Barrier: 60%
Unstable Warping Arm guard: 75%
Jagged Decaying Shield: 60%
Spiked Stone Wall: 60%

(I think immortals shield gives spell damage reduction as well im not sure on the % I would guess that its 80%-95%)

Moving on

lets say you have cloak of the unseen 50% spell damage reduction weak con is level 1 it still reduces your spell damage reduction to -20%.
just to enforce my point lets say you have immortals boots 95% spell damage reduction against a level 1 weak con your spell damage reduction is still -20%.
now im going to go out on a limb and say Magekiller aura works the same way as item spell reduction so even if you have it maxed out a level 1 weak constitution makes your reduction -20%.
Now lets factor banish in.Though Weak con sets spell damage reduction to a set value banish still reduces it.So lets say you have Weak con level 2 and banish for say 25% vs someone with Magekiller for say 50% and say skeleton army.Your Weak con reduces his summons from 50% to -25% then you cast banish on his hero and his spell damage reduction is now -50%
in turn spells are going to hurt.

now for banish and regular spell damage reduction they will just cancel each other out say banish is at say 30% and you have 50% spell damage reduction 50-30=20 there you go.

Any questions ?

I will just include here 3 small factors.

Banish is an armor type, Ethreal. It increases spell damage the same way fortified armor takes extra damage from siege. This is not linked at all to spell damage reduction or amplification from anything else.

The 20% that heroes have as a base is not added into the equation for anything else; this again, is an armor type deal. It works the same way as banish. When you are hit with banish, this 20% is also taken away, because your armor type is changed.

the 50% on an item does stack with the 20% base your hero has. This is how it works: you only take 80% spell reduction normally. That 80% is then cut by 50%; which is actually 40% from the 80% (50% of 80 is 40). Making your total spell reduction 60%.

I actually believe that mage killer aura mightttttt override weak C; not positive. needs to be tested. They might also just cancel each other out.

Well thanks for clearing that up we will have to test Magekiller vs Weak con.To find out how thats going to work then.