I play him as a tanky an agressive tank I usually play top with the carry or a ranged character.His insane damage is well used when you can get those stacks fast a crippling strike and swoop them back with my pull then exhaust if by now their not dead use your ult and their toast:)

I use these items for sustainability and movement which Darius needs very badly to deal out all that true damage.

Attack marks for that extra heavy hit early game.
Armor yellows for the sustain
Flat cool down glyphs to spam your abilities more his pull is a long wait and is very useful for chasing those squisheys
and finally movement quints no explanation needed.

Your probably wondering why I chose those 9 in offence here is my reasoning. For the improved ghost and exhaust and cool down plus 4 extra ap for his passive. The 21 in defense for the tankyness and cooldown.

I get q first for the most damage output when laning and learn to judge his swing so you can do extra damage with it .i max w by level 14 for the slow and extra damage it will do the next attack you make I max the e last just because its not too usefully only to pull them in and armor pen not really a damaging ability and of course ult when possible and try to max stacks of bleed to add the extra damage.