First off, it's how you Build/Play Darius.
I like the Dyrus Method, in which he rushes The Black Cleaver and then goes Full Tank. Of course a Sterek's or Titantic can be added.
The reason being is you don't need actual damage... you just need to get Stacks then Ult, and already have a ton of Armor Shred.
Now, what works with him
Darius will be a Front Line Tank in this Set Up, and will look for chances to get his Pull on a Carry to Delete. When not finding a Pull he'll simply use his Tank to keep standing, his Q to feed him Life Steal, while his Armor Pen will slowly eat Tanks. So keeping what you are trying to do... Stand as a Front Line and look for a Dive to Peel or a Carry to Catch... you know what you need around you.
Pretty simply, you need some CC to lock down Targets for you to get there. After all, you don't want to walk past the Front Line to Pull an ADC that Flashes away, and leaves you on the wrong side of the Ugly Stick.
Also AoE is your Friend, since you'll be landing some AoE with your Q, and with your Hydra if you go that route (or Sunfire, but less popular).
Also you WILL be focused, no matter how Tanky you are people see Darius and decide to try to delete him (with good reason, Burst Darius can delete anyone and Tank Darius can do insane Sustain Damage and Execute). So Shielding/Healing is very helpful.
So think of a blend of CC, AoE and Heal/Shield. You can achieve this in a variety of ways. Ziggs Mid gives more than enough AoE, while Sona can give Heals and Nautalis or Sejuani in the Jungle can provide plenty of CC (and AoE). Just being one example.
Another Champion he works well with is Sivir in a Run Down Comp.
So just remember, CC, AoE, Heals/Shields are all good for you. A Shen to Ult on you as you Engage (though Defensive Darius really Counter Engages, and Dyrus does this amazingly well, even better than being Offensive). A Leona to start a CC Chain that you just wreck into. All kinds of things.
The good thing about Darius is he can fit in a lot of different Comps.