The way the new armor pen works, If you have armor pen runes and 2 armor pen items, basically all of darius's attack/skills will do true damage to everyone except tanks.
%armor pen maxes out at 75% but new last whisper is "ignore armor" not armor pen, so darius practically ignores 400 armor on everyone.
Here is how
The Black CleaverCost: 3000 (1188)
+50 Attack Damage
+10% Cooldown Reduction
+15 Armor Penetration
Passive: Dealing physical damage to an enemy champion reduces their Armor by 7.5% for 4 seconds. This effect stacks up to 4 times.
7.5X4= 30%+15 flat penetration
Last WhisperCost: 2135 (860)
+40 Attack Damage
UNIQUE Passive: You ignore 35% of your opponent's armor.
so 25% armor pen (passive E) + 8% from masteries 30% armor reduction from Black clever + 35% ignoring opponent's armor
That means you penetrate 33%, reduce 30% and ignore 35% which is 98% of armor negated. =\
LETS SAY you have 90% armor pen/reduction/ignore(total) instead of 98%
you have flat 15 from Black clever and with runes you have about 25 that is about 40 armor pen (flat)
IF you have 400 armor, take off 90% which will leave you with 40 armor, and with 40 flat, you have 0.
so darius does true damage to you even if you have 400 armor =\
Fix darius soon!