As with any champion build, this one works only for specific situations. Dont be afraid to improvise. I've been using this build with huge success playing Dominion and not going bottom lane.

Greater Mark of Insight - Because magic pen is amazing. Since I usually don't get Sorcerer's Shoes, its nice to have that little extra punch.

Greater Seal of Clarity - To take full advantage of the sustainability the spell vamp offers, we need to get our mana regen up as high as possible as well. I oftentimes stay out from the time I buy my Will of the Ancients until I can afford to buy Rabadon Deathcap from scratch, but thats only possible because I can cast spells for days.

Greater Glyph of Force - I like Greater Glyph of Force because AP. Nothing sucks about AP. The only other real option for this slot is Greater Glyph of Potency, but I'm a huge fan of scaling runes in Dominion since you start at level 3 and end up spending 2/3 to 3/4 of the game with a greater benefit than from flat runes.

Greater Quintessence of Transmutation - Between these and Will of the Ancients, you get 31% spell vamp all the time, plus an additional 35% when Soul Eater procs. I've found spell vamp to make or break Ahri, so get as much as you can. The only other options I've found viable are Greater Quintessence of Force or Greater Quintessence of Potency, but in the end spell vamp is much harder to come by in game via items than AP, and I can't stress how useful it is to the build.

This one's pretty simple. With Ahri the best defense is a good offense, so you need straight face raping masteries. 21 points into Offense should be a given. I've done well going 21-9-0 and 21-0-9 but prefer the latter. Due to the fact that spell vamp plays a big part of this build I'd rather have the scaling base mana and mana regen over health and bit of armor and magic resist. The point into Runic Affinity is pretty good too since it applies to speed buffs, quest buffs and the storm shield.

I start off with Boots of Speed and two Amplifying Tomes and then rush Will of the Ancients, picking up Ionian Boots of Lucidity either after the Hextech Revolver or once Will is finished. Once I've got my sustainability set, I recommend Rabadon Death Cap. Why? Because otherwise he doesnt hit hard, and thats not very fun. Next I get Morello's Evil Tome for the CDR. After that, its really all about what you need most. Rylai's Crystal Scepter, Void Staff, Abyssal Scepter and Zhonya's Hourglass are all good choices.

Some of my teammates think that its better to get Spirit Visage between Will of the Ancients and Rabadon Deathcap. While the 15% increased health is pretty good in combination with the spell vamp, I find that I do better by just gaining more life by doing more damage.

Orb of Deception is definitely your hard hitter. The AP ratios apply on both hits and the second hit deals TRUE DAMAGE. On top of that, the cool down is reduced every level. Max it first.

Fox Fire is pretty good too and I have a feeling that most builds will level it second, I however prefer to level it last.

Charm is way better when it lasts for two seconds than one. I know it slows down your DPS to level this before fire fox, but I think its worth it. Hitting someone with a one second taunt is good, but a two second taunt can pull an opponent off their tower and straight to their doom.

Spirit Rush should, like all Ulti's, be leveled up whenever possible. Its an awesome escape if you get caught out of position, but its true strength comes in team fights. You can easily burst three champions down while remaining relatively hard to hit.

The most frequently used combo is Charm, Orb of Deception, Fox Fire. Hitting an opponent with Charm pretty much guarantees a double hit with Orb of Deception and as many hits as you want with Fox Fire. This is your most versatile combo. It works well when fighting alone, as you get two seconds to go through your burst rotation and either run away or commit before your opponent gets a chance to do anything. It also works well when capturing or defending a tower by forcing an enemy to walk away from the safety of their turret or into the range of your own. In team fights, it often feels like charming one enemy champion causes all of them to come straight at you. This is great, because hitting multiple enemies with Orb of Deception doesnt decrease the damage dealt per target. The more the merrier!

There are a few situational combos that involve Spirit Rush. In a 2v1 fight, I like to activate Fox Fire, Spirit Rush between the two champions (hitting both with Spirit Rush and Fox Fire), throw Orb of Deception in one direction and then Spirit Rush in the other. This causes both champions to get hit by Fox Fire, Spirit Rush (twice!) and the true damage portion of Orb of Deception, as well as the extra third Fox Fire hit and the first Orb hit, while saving the third Spirit Rush as either an additional escape or a finishing blow to both enemies.

This is my first attempt at a build, so be nice. I couldn't figure out how to make it so all the item and ability names would link to the appropriate pages, if someone would be so kind as to tell me how to do that I'll get right on it.