I chose 9 21 0 masteries because:
1. Summoners Wrath
- +5 ad which after using Ignite is useful early game and makes your harass even stronger.
- your autoattacks to stack your passive faster also it's useful for last hitting.
3. Weapon Expertise
- With new percentage of armor pen, new 8% is BETTER than old 10% so this mastery worth 1 point but it's very useful ( with your E's passive it's even better).
- sustain in lane and some free health regen is always good.
and Veteran's Scar
gives you early 30 hp makes you look even scarier (works well with Juggernaut). Darius should be tanky and more HP is always useful for him.
- a little win while trading with autoattacking champions. -5 damage from each autoattack is great.
- best utility for darius. If someone will take a kiting champion like jayce, kayle, teemo or even lulu to counter darius - this mastery is your best friend. It makes darius able to close gap to do your combo. Except laning phase this mastery gives you survivability and movement.
- 104% of your maximum health instead of 100% which is extremely useful, especially if you've bought any HP items.
9. Feel free to use Good Hands
mastery even if you feel confident in lane and you are always aware of the map. Mid to late game this mastery is very useful.
10. Honor Guard
- fantastic mastery versus burst damage champions. Instead of 100 damage you will take only 97 (with 0 defence). It scales very well with defensive items.