Hey,
welcome to my first build.
This is my way of playing Ahri, which works out really well for me.
With this build her survivability is higher and she will do loads of damage.
I am open for critique so if u have please let me know.
Thanks for reading my build and I hope to see your comments below.

Pros
- Great damage.
- Escaping ganks is pretty easy with your R + Flash. Even though I rather use it to pick up a kill.
- Ganking is pretty easy. (With your E, if it hits.) Just make sure they don't focus u.

Cons
- Quite squishy!
- Runs low on mana. (But you can prevent that with this build.)
- Targeted in team fights

Greater Mark of Magic Penetration
All of Ahri's abilities are damaging spells. That's why Magic pen. is the best choice.
Plus you can only get a small amount from items so to still get the magic resistance reduction she needs I use these runes.

Greater Seal of Scaling Mana Regeneration
You will find yourself running low on mana many times without there runes. Combined with Doran's ring I buy twice in early game you won't have that problem all the time.

Greater Glyph of Scaling Cooldown Reduction
Cool down reduction runes work really well on her. The more u have the more abilities u can throw out. Ability power Glyphs are optional too but I feel like she does enough damage already with AP Quintessence

Greater Quintessence of Ability Power
This will give her some good amount of damage to go with her abilities.