I only took much Armor Pen. because Darius like to SHRED the enemys Armor because of his E - passive and of his R - true dmg. With Black Cleaver this is a dream.

U can take Dorans Blade when u would like to play aggressively and Dorans Shield when u play passively or against a hard enemy.At the first recall u can buy Brutalizer and then Ravenous Hydra for the AOE with ur W.After that u upgrade ur Brutalizer into the Youmuus Ghostblade.At the end of ur Core u buy Trinitys Force (Sheen.Phage and then Zeal).The Unique Passive Spellblade is cool because at lvl 18 u do 230 dmg more with ur W
(BASE AD *2).Then u can buy Bloodthirster for Life Leech,Black Cleaver for Armor Shred or IE for more DMG with ur W.U can buy Tanky Items,too e.g. Spirit Visage,If u buy BT,and u have fat/fed APC´s as enemys or Banshee´s Veil if u have a strong Burst in ur enemy team (like LB or Nidalee).If u need much Armor,U´ll buy Sunfires or Randuins Omen.Do ur enemy fed u and u r rlly strong u can buy Sword of the Occult.I don´t like shoes so i buy they rlly late if u have stong cc/Apc´s u´ll buy Mercurys and vs. fat ADC´s Ninja Tabis.When u would like to see real Dmg buy Last Whisper.With Last Whisper u and a full BC u do have 90% Armo Pen+39,20 with Youmuus and Runes(at 100 Armor u shred 129,20 Armor,at 200 u shred 219.20 armor and with 300 armor 309,20 armor)

In ur laning Phase u skill ur Q with much Dmg and then ur W cuz of DMG u can skill ur E at lvl 2 when u need the hook.