Hello, and welcome!
Im playing Diana since PBE, and I'm in love! I love her dress, i love her jokes, i love her intro, graphics, animations, moves, EVERYTHING!:)

She is real Battlemage! Diana prefer close fight, but still all of her attack's should depend on AP. She has extremly high burst dmg, very good harasment, and if she catch you in 1vs1 you probably die.

I prefer this build than 21/9/0 becouse of mana boost and movement speed on utility tree, i think i dont need to explain why 21 in ofence tree:)

Summoner's Wrath: Worth more then both Brute Force and Mental Force combined when you use Ignite. Definitely a must.
Alacrity: Just a tiny increase of AS to boost up your passive
Mental Force: A little extra 3 AP to help you last hit.
Sorcery: Reduce Cool-downs by a pinch.
Arcane Knowledge: 1/4 of a Void Staff's passive.
Blast: +18 AP at level 18.
Archmage: Definitely a must to dangerously increase your damage output.
Executioner: Become an assassin, get your kills here.


Summoner's Insight: Reduce the cool-down on Flash for those times you will need it.
Expanded Mind: Need that mana per level. Allows you to spam your abilities more.
Improved Recall: 1 second out of 8 seconds gone. You now get out faster.
Swiftness: Better fast then slow. Helps throughout the game.
Runic Affinity: Blue Buff will last much longer now.

Skill's... so first things first!

Her Q Crescent Strikr is the best thing i ever seen! this curve shoot is so nice, that if you get some skills with her you will only see opponents movements in curves. You will use it so good, that you will catch 4-5 champions in TF while harrasment time. It has anourmous dmg while you get Rabadons, it must be maxed first to get even more dmg!:)

Her W is very nice though! Dont use it like a shield, its more like another close range attack with a little bonus. Max it as a second skill, it has nice dmg, and it helps a lot in close range fight or while runnning from opponent (it will save your live while someone will ignite you).

Her E is so nice, that i still cant believe it. Its Orianna's ulti effects with slow for 2 sec! This is so beautiful while chasing opponent and he cant run from you (Q-W-R-E-Q-R-R and you have kill!). Take it as 3rd and max it as third beacouse it wont change a lot while upgrading it.

Max her R as fast as possible, it has ability to chase down everyone, and if you running from opponents try run to jungles and use combo Q-R on minions behind the wall its free Flash.

Ive tried many builds, but this one i prefer the most. I dont Focus on passive (which is still very good and i dont forget about him) but on AP maxing Q,W,R,and passive effects.

Im starting with boots and 3 health potion (better sustain on lane), its much better than dorans, becouse u can stay on lane with no problems till lvl 6. She has good harassment with Q, and i use it only for this purpose, becouse i will oom quickly. Kill minions with melee.

Secondly i got for Rylai's (for HP and AP boost with a little bonus (slow)). I take blasting wand at lvl 6 and belt if i have enough money. At this point i starting my assasination. Trying to attack opponent which has about half HP (its enough to kill him easly). Or if you have jungler to help, this will be very easy.

As next item i prefer Sheen (for his passive, AP and mana), but if your team is loosing, and you need to help them, or if you feel that you need mercurys treads first - go for it.

As a priority on Diana is Rabadons hat! Everytime, when you go back to base, spend your money to increase your AP, so buy Blasting Wand or Needlessly Large Rod (or both). While you get rabadons, your attacks will destroy.

Then i go for Nashor tooth (for better use of your passive) 50% AS, 65 AP, 25% CDR and 10 mana regen is soooooo goooood for Diana!

As next item i go for Abbysal or Zhonya's (its matter of they have more AP or AD users) if its euqal i go for Zhonya's beacouse of 2 sec stasis which can save your *** at very bad sittuations.

Thank you guys for everything, if you see something wrong, or if you want me to help to improve this build please share your comments below!

Sorry for my english.... :)

Sincerely,
Ashinron