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=NOW GO SMASH

PROS
+ High damage output
+ Decent lane sustain
+ Tank buster
+ Great lane phase in most match-ups
+ High snowball potential


CONS
- Easily Kited
- Little mobility
- Reasonably squishy

For your summoners you will always want to take Flash It is really good as an escape ability if you get caught and can also be used offensively to get to your target to secure a kill.
For your second summoner I would recommend Ghost, Teleport or Ignite.
I take Ghost almost every game as it's very good for extra mobility to chase down your lane opponent or for using in a team fight to get to the back line and destroy the ranged carries.
Teleport is a summoner I take only if I'm planning on either #1. Split pushing all game 2#. Teleporting bot lane to get a lead for my team.
I only use Ignite against certain champions such as Yorick or Fiora just because I am bad and lose most 1v1's without it.

It's really up to you and your play style but I highly recommend you take Ghost as your default.

Farming is the first thing to focus on in lane as Darius. Use Decimate for wave clear and Crippling Strike to help last hit.

All throughout the game you should be looking to shove lanes and create map pressure. You can either take towers or fight with the team depending on the situation.

You have two main options as Darius in fights. You can either A) Fight the frontline and peel for your carries or B) chase the backline and try to destroy the enemy carries. Taking Ghost will help a lot with option B.

Try to focus on a target you think you will be able to get 5 stacks of Hemorrhage on easily (such as a tank or low mobility champion) and then you will be able to deal a lot more damage for the rest of the fight.
Use Apprehend to reel in squishy targets and slow them with Crippling Strike.

Once enemies are low use Noxian Guillotine to execute them (provided you have sufficient amount of stacks of your passive on them)