I use 9/21/0 for some damage and tankyness to provide you with not only enough damage to kill enemies quick but to also survive them if they decide to focus you.

As Darius you would want to max you (Q) as soon as possible and after that you (E) so that you have armor pene and your (Q) to devastate your enemies health, your (W) does not do much damage but is useful for it's slow thats why i max it last. As for you ult put a point into it when ever possible.

I choose AD Seals, Glyphs, and Quints for the extra damage so that you can take early advantage towards the the opposing enemy. I use Armor pene Marks so that all that damage from your other runes is put to good work. I use Strength Seals but Mobafire did not have them as a choice so please keep that in mind.

For your items you dont need to build much damage just really tanky because darius already does so much damage that all you need to do is hit the enemy a couple times until they are around 3-5 bars and then ult them to kill them and have your ult ready for use again.