Welcome to my first build/guide.

First off this is just a short guide, fleshing out the details on the build.

Items
Runes
Spells
Gameplay

For quick specifics, this build it meant for APC Mid/Roamer.

Some people will instantly see Ionic Spark and think, "why". For my idea, this is incorporating all the facets IS brings to Diana. More Attack Speed for faster passive procs along with it's own passive proc, along with the extra HP she becomes just a bit more tanky.

Obviously this can be swapped for Malady if that more suits your playstyle.

Nashor's Tooth might also catch your attention but this item was practically made for champs like Diana IMO.

Rylia's, Rod, and DC are 3 items you should be rather aware of the reasons for getting as a matter or course.

If you're really disliking the AS swap Nashor's Tooth for a Lich Bane and Ionic Spark for a Banshee's Veil.

Q - AP/Level*3 or Magic Pen*3
M - Magic Pen*9
S - AP/Level*9
G - AP/Level*9

I went full AP/Level with Magic Pen marks for overall endgame damage with some good increasing damage as you level.

So, Teleport. Believe it or not this spell, if used correctly and in conjunction with your team and their wards, is indispensable. To quickly, and hopefully without notice, be in the bushes of top/bot for a quick gank. Great if used right after lvl 6.

Exhaust is perfect for securing that kill that you really need to make sure goes down without much fuss. Ignite is so popular that I've seen full teams running it, it's good to have a mix and so because Exhaust works well with her kit... that's the obvious choice for me.

Broken into two parts.

Laning Phase:
Essentially your job here is to farm but not feed. She's not great against a lot of typical Mid champs simply due to the fact most of them are 100% ranged. You've gotta pick your battles and be aware of your surroundings. Once you hit 6 base, buy what items you can and ask for someone to drop a ward in the nearest unwarded (by the enemy) bush. If all goes as planned you can give a little hand to whatever lane is having problems if any or secure dominance in the lane that's already winning the balance.

Team Phase:
You have 1 job in any team fight that first breaks out. Kill either the ADC or APC. Usually go for the ADC since they often are the most squishy with least amount of MR. If you're really good a typical skill sequence Q W R E (once they start trying to run) Q R R if they're not dead by this point then something's gone horribly horribly wrong, and you're probably about to die.

So that's it in a nutshell, I won't be writing any in-depth tutorial/guide for playing her since my playstyle might be vastly different from my own. Post any comments or suggestions, I will happily look at them and consider it.