Olaf's armor pene passive was changed to an active

Yeah, I was gonna say. His ult is a trade off. You sacrifice defensive stats for offensive stats.

You gotta build armour pen yourself.

Although if you play right and build Lifesteal, Olaf counters Darius. He does well against him early, but later Darius is trouble. At least Olaf can keep Darius from an early snowball, if you are a seasoned Olaf player.

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Olaf has 0 armor pen in his kit. Has been that way since his soft rework a long while back.

before his rework, before he was olafed he had an armor pene passive which was changed to AN ACTIVE because it was broken. Once he hit 6 his Q used to deal an giant amount of damage.

Darius is basically the same with his E, at level 9 he has 10% armor pene, if you have 100 armor it's -10 which used to be olaf passive