This guide is not as finished as I would like it to be! Gonna upload anyway and try to work on it as much as possible!
Adding more detail/information/etc. Just give me some time!
Feel free to leave feedback and what you think!

1.11
Added Honorable Opponents (will add more soon)
Added Items (again, will increase this with more items and reasons why you should use them)
Updated Pros/Cons
Added runes/masteries (will go further into explanations soon!
1.15
Updated additional Item Reasoning
Touch ups
1.19
Added Skill Sequence.
Additional touch ups.
Added Farming section.
Updated builds. Added 5's build!
1.21
Added Abbreviations
Updated Spells
Additional Touch Ups
Updated Honorable Opponents
7.18
(I'd like to apologize for the huge break between updating this guide. It wasn't my intention and i'm back to making guides/updating this one)
Updated Guide to Darius Nerf, updated items, updated content.
7.21 Updated Example Build / Items
Updated Reasoning Behind Items.

Cons [-]People Cry when you play him [-]Mana can deplete rather quickly while ulting [-]Little Escape [-]Gets focused early game

Pros [+]Dangerously Strong [+]Tanky as hell [+]Can Carry your team [+]Spammable Ultimate (if used within 12 seconds of first use)

Ghost:

I think this is one of the most crucial SS for a viable Darius. Firstly, it promotes mobility either for escapes or kills. Use this when chasing or getting away. Good to pop so you can quickly move around target for those Decimate. (The outer area of decimate does 50% more damage so it's great to be able to move about and get that hit on.)

Ignite

I like to carry this bad boy around for those close call champions. Reduces healing on them (comes in handy for early top lane kills) and does true damage! What's not to love?
You can swap this out for Exhaustif you'd like.

Also, feel free to use other summoner spells. It's always about personal preference. If you enjoy having flash, take up flash. These are just in my opinion great summoner spells to use.

Feel free to swap out Destruction for Weapon Expertise . I mean if the opponent is loaded with armor it might be useful. Though with the items in this build and Dariuss abilities, I don't think you need the 8% armor pen.
Keep in mind, these are just some talents I use. Feel free to explore and use whatever talents you'd like. If you don't want the tanky talents, you can refer to the Brute talent tree I have posted.

• Greater Quintessence of Attack Damage: I use this to add some needed AD, helpful early game and if you aren't using Ghost I would recommend getting movement speed quints.
  
• Greater Mark of Attack Damage: Again, use this for extra attack damage, since this build focuses mostly on hp.
  
• Greater Glyph of Magic Resist: I like to use this to add the extra needed magic resist, can never go wrong with this one. You usually always face some AP.
  
• Greater Seal of Magic Resist: Same reason as above, this allows some extra MR early game and let's you build health early.

Atma's Impaler grants 15 attack damage for every 1000 health, making it the most cost-effective on champions who build very large health pools. This particular build focuses hugely on large HP. Having 5k hp on a Darius with this item is very intimidating.

Overlord's Bloodmail Grants 850 HP. On kill or assist champion 200 hp over five seconds. This is particularly useful when you are being focused and you land a Noxian Guillotine.

The BrutalizerStats +25 attack damage
Passive Unique: +10% cooldown reduction
Unique: +10 armor penetration
Very helpful when stacked with Darius' Apprehend ability, great against heavy armor wearers.
Black Cleaver
+250 health
+50 attack damage
+10% cooldown reduction
Unique: +10 armor penetration
Passive: Dealing physical damage to an enemy champion reduces their armor by 6.25% for 4 seconds. This effect stacks up to 4 times.
I highly recommend building into this. It provides the armor pen and the health.

Yorick + Dariususually equals a struggling Darius. Yorick is in my opinion one of the best champions to face Darius with. Darius has low escape, (has to pop summoner spells) and Yoricks ghouls and abilities just eat him up sometimes. Use your team to help crush this opponent.

Jayce is another opponent up there who can really bring the fight to Darius. He is a brute with great mobility and utility against Darius. Play it safe and use team mates while against a Jayce.

Kayle is another strong opponent against Darius. With her speed increase, and slow. She can range Darius down to a pulp sometimes. I recommend playing it safe and utilizing your team while against this champion.

Teemo It's Teemo, so he is pretty strong against many a characters, including Darius. Avoid pulling him in and getting Blinding Dart. If you need to, get your team to help out an early gank, one kill ahead is really a game changer.

Xin Zhao Is a great champion to have fun with against Darius. His Audacious Charge is a great harasser and allows him to help out his team with ganks. I'd suggest calling out ganks on this champ early game, at least get him to recall. Get a kill or two ahead and you'll be good.

Malphite Play it safe, don't let fight if he has his slow on you. Ground Slam. If there is someone on your team more viable to face him I'd suggest switching up lanes. His ultimate hurts and can be used to help you get ganked.

Skill Sequence!

• Apprehend: This is a very helpful skill to use when initiating a team fight or a solo fight. It pulls them to you while granting you passive armor penetration! I suggest using this unless the enemy has jumped to you. If the enemy has jumped to you first. I suggest throwing on Crippling Strike. This slows their attack speed and movement speed. Then as they try to run away you land a Decimate on the outer blade for 50% more damage and then pull them pack with Apprehend. They shouldn't stand a chance.
  
• Decimate: You can argue or not to use this next, or Crippling Strike. They won't be slowed if you use this first. Usually an enemy when they pull them (which is your initiate move Apprehend, their first instinct is to back away. This lands you the perfect opportunity to land a Decimate with 50% more damage. You then usually can slow them with Crippling Strike and by then your Decimate will be off cool down. And you then should be ready for a Noxian Guillotine.
  
• Crippling Strike: Like I stated above, you can argue whether or not to use this second or third. Also, please keep in mind, the game is situation based.. So if you pull them into and think they are easy to get away, I'd throw a Crippling Strike on them first, then land the Decimate. This is also why I threw in Ghost as one of your SS. So you can usually catch up to an enemy.
  
• Noxian Guillotine: Many Darius' do NOT know how to utilize their ultimate. I've seen so many use it too early, and end up turning the tides for the enemy team. I want you guys to pay attention to your Hemorrhage stacks and keep in mind your Ultimate's rank. Many a times, people don't grasp the concept of how many stacks of their passive is on their enemy. You can stack this bad boy FIVE times. For 100% more damage. This can turn a fight, and you can take out a third of an enemy's health. One of the greatest things about this is that it is true damage. One of the only ways to counter true damage is to build hp. Many opponents will be victims to this ability.

Farming on Darius can be tricky at times. Especially against a ranged laner. Getting last hit is important for your CS, but it's not worth a death or being poked to death.

There is a time when you should use Decimate on creeps. This provides you with extra range and allows you to trash some creeps and grant you higher CS.

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Please keep in mind, a smart enemy who sees you use your Decimate will sometimes leap on you knowing your Decimate is on cooldown for a few seconds.

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If you are struggling on farm because the enemy has range, utilize your team, call out a gank and happy farming. If you can at least get the enemy to recall. There is a nice opportunity to farm your CS.