Well, AHRI: The Nine-Tailed Fox is a champion killer AP. His skills have a high level of AP and are destructive if used at combos. It was launched in League of Legends on December 14, 2011. She has a good farm, high damage and high fun.

Greater Mark of Insight - Magic Penetration makes at the beginning of the match your Q becomes totally destructive, from your true damage. Almost all true damage.
Greater Seal fo Replenishment - Mana regen will make your mana has a longer duration, so you can use skills to kill and to creep all the time.
Greater Glyph of Potency - Start game with an advantage of almost 30 Ability power. It makes your skills are too strong.
Greater Quintessence of Potency - Along with the Glyphs of Potency is the beginning of the game with an advantage of almost 30 Ability power.

I particularly prefer the power cast often the advantage of ensuring early game. Then, use the masteries 9/0/21. Many mana regen and cooldown reduction. But an alternative is using 21/0/9. Matter of opinion.

Boots - Boots and potions to greater sustainability in the lane or mid.
Catalyst the Protector - Catalisty makes your mana and life are never down.
Sorcerers Shoes - More magic penetration, reducing the enemy's magic resist.
Hextech Revolver - Revolver combo with his passive, very good to regenerate life
Rod of Ages - More mana and life, meets the need for sustainability of Ahri.
Rabadons Deathcap - More than 30% of all power Ability. Logical.
Sheen - Bonus in the attack, more a complement to the skills. Later becoming Lich Bane.
Lich Bane - Attack bonus with all his power Ability, the more damage using basic attack combo.
Void Staff - Much magic penetration, almost total true damage.

At the beginning of the match AHRI realação is very weak in defense and life. But very strong in their skills.