I was thinking about this whilst I was in the shower, and I was wondering. Would a champion with a true DMG reducing passive be too op?
Since you have 
being the true dmg dominators, and the introduction of the conqueror rune, it would be very interesting to see a champion with a passive like that. A tank of course where their passive scales off some armour and magic resist. Since they are tanks I don't think the scaling would be too high.
It makes sense to have the dmg reduction scale off of armor and magic resist. There should always be a way to bypass resistances. With true dmg being reduced, the ADC or Mage can still shred with penetration.
What if the passive was an item? Because Conqueror can be troubling for any champion caught in the above champions' paths. There is absolutely no counter to true damage in the current game, and with it being so highly available now, the counter should be too.
How about something this: Aegis of the Legion (1100) + Glacial Shroud (900)
+50 Armor +30 Magic Resist +300 Mana +10% Cooldown Reduction
UNIQUE Passive: True damage taken is reduced by 25% (+1% per 10 Bonus Armor, to a maximum of 90%).
... It'd be horrible, because suddenly this button, whose entire point is "you always know how much damage this deals", no longer has that.
cool idea, but it seems true damage is put here as such with reason and therefore balanced as such to serve the purpose
"True damage dominators", ahri's backdraft on her q shouldn't put her on the same level as Cho'Gath
My gripe with this kind of concept is that you counter a handful of champions, but this has no effect on any others. It would be good against the string of Champions you listed, but again, doesn't have any real effect on people such as, say, Kha'Zix, Zed, LeBlanc, Lissandra, Veigar, etc. because it only reduces True Damage. Straight up damage reduction is boring, as well; it's a passive, sure, but you don't really notice anything. TL;DR: It's too niche, and too dull.
that would probably be good to be seen as a compensation , where that champion would have some of his armor and mr removed in exchange for ignoring some amount of true damage, looks healthy
