Hello, I am not one of those pro players, nor am I a regular Mobafire user, so there may be a good chance that this guide is not for you. However, if you are willing to try it, I can give some explanations of why I chose what I chose and how it will benefit your game.

At first look their may be a few items and/or selections that you don't agree with. Such as the selection of Malady over the use of Nashor's Tooth, or the use of surge, or the use of 25/5/0 in masteries. This guide is meant to be flexible with substitutions from other possible items, runes, or masteries. The choice should vary based on the game, the player, and the style of the player.

Diana is decently versatile and a very strong AP champion. She can be played as an AP bruiser, or a glass cannon. This build is somewhat of light bruiser build for her and gives her some durability that increases her usefulness in prolonged fighting. However a majority of the emphasis is placed on her magic penetration and attack speed. These two stats are very important for high damage output on Diana. Pure AP allows for powerful abilities, but cool down hinders her ability to finish kills or assist in team fights. I recommend that attack speed and durability give her the necessary stats to be a useful and valuable part of the team.

Pros:

+Great Harass Ability
+Ultimate is a good finisher
+Passive is amazing for farming
+Fantastic Burst Potential
Cons:

-Requires decent amount of CS or feeding
-Subject to being focused by cc and carries
-Does not have an official escape mechanism

The suggested runes are focused to cover a few key stats areas that need strengthening. I will start by giving the suggested runes and possible alternatives that I considered. These may not be your optimal runes but they can be very useful and should be considered.

Marks:

First, Magic Penetration Marks are always useful. The reduce the defensive stats of enemies to increase over all damage potential. They are always worthwhile. I recommend them so that you can significant damage to both tanks and to squishies. You can never have too much penetration. The reason for these runes is so that the abilities and magic damage output from the passive are not greatly diminished from enemies' resistance.

Alternatives to consider might include attack speed runes (flat or per level). These would be my second choice. They are the most useful stat to bolster after magic penetration. These would increase the speed at which your passive will activate and can increase damage potential even farther. If you were to chose these, I recommend that you select attack speed per level marks, their usefulness increases as game time progresses.

Seals:

I recommend the use of flat armor seals for this Diana build. The reason being that Diana is a squishy and needs some durability to survive early game so that she may thrive in the late game. Armor per level seals would also be a possibility, but Diana may run into troubles before their stats become significant.

Alternatives to consider would be flat health, health per level, armor per level, or even mana regen per level seals. I would personally recommend defensive stats over health or mana regen. The Rod of Ages gives access to plenty of mana that should prevent the need for regen. But if you feel like you need more, you can choose the runes that do so.

Glyphs and Quintessences

Both of these fall under the AP stat and so I will cover them together. I will start by explaining their function. AP per level glyphs are useful for adding to the late game damage output that is necessary for an effective mid to late game. They are useful, but are practically worthless in the beginning game. To make up for this I suggest that Flat AP Quints be used to bolster the AP output for Diana. These Flat AP Quints are the equivilant of having the AP of a Doran's ring. Thier function is to help Diana by giving her the damage to be a threat in her lane and boots give her the mobility to finish targets and escape threats.

There are a variety of different possible combinations that could be used in the place of this one, but I recommend AP per level glyphs and Flat AP Qintessences to give Diana effective early and mid/late game stats.

Most of these are pretty self explanitory. I would recommend any that give you bonus stats in AP, Attack Speed, or Cool Down. The additional 4 points placed in offense mastery go to alacrity for attack speed. Once again this is because I made her to be a light but durable bruiser capable of high damage output. For defense, I chose to add points to mag resist and armor. You can choose health as well, but I prefer defense to life.

With Items I focused primarily on bolstering the attack speed, ability power, health, and magic resist of Diana. These stats are significant for damage and durability. I will explain why the items I chose are useful for Diana and their intended purpose.

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>>I know I suggested that you can never have too much magic penetration, but Beserker boots play a vital role in this build for Diana. Their increased attack speed increases the speed of the passive and helps farm minions without using mana. Other items cover the magic penetration and add to the overall AP stat. I recommend that you select Beserker boots over that of Sorcerer boots.

>>Rod of Ages is the next key item. This item is a crucial part to Diana's build. It could be substituted for with Rylai's Crystal Scepter, but it's large amounts of Life, Mana, and Ability Power make it an incredibly efficient use of an item for Diana. Plus it's passive gives sustain that Diana needs to stay in lane.

>>Next isMalady. I never really considered this item very useful, but it is everything that Diana uses and needs. Additional attack speed, ability power, and magic resist reduction make it a worthy choice, and it's cheapness makes it an easy to attain item. This item is great for farming minions. With the increased attack speed and ability power enhancing Diana's passive, they make it easy and quick for Diana to farm without the use of mana. This item's unique passive greatly helps when fighting champions with magic resist. When this item reaches max stacks, enemy champions lose 24 magic resist for 8 seconds. Coupled with Magic penetration runes and percent magic penetration mastery this item becomes a very useful tool in bursting champs down. (Note: The Magic Resist reduction benefits the passive damage output, ensuring that less damage is blocked for higher output) Nashor's tooth is a good alternative to Malady. It's AP is higher, it grants higher mana regen and has cooldown reduc. It is an excellent choice but the magic reduc. is too attractive to pass up. This choice is up to the player.

>>Rabadon's Deathcap is an essential part of any AP champ's inventory that is seeking to do significant damage. I would really consider this a necessary core item to Diana and should not be replaced.

>>Abyssal Scepter is the perfect item for Diana. With her Ult she can focus a single champ or go into a team. The unique passive of Abyssal Scepter grants a field of -20 magic resist that assist both you and your team's AP champs. It is a potent item.

>>Lichbane is the final item. Since Diana relies so much on AP and magic damage based on physical attack, Lichbane is an excellent choice to boost damage output and increase chasing potential. Her passive strikes around 600 when fully built and Lichbane uses the 600+ AP and turns that into more damage output.

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Considerable Alternative, Substitution, and Enhancement Items

>>Rylai's Crystal Scepter- This item is good for hitting a target with Lunar Rush to apply the slow debuff, after which you may proceed to reduce their magic resist and pummel them with ap damage. This is a decent replacement and could be useful for enemies that have a good amount of movement speed on you. I caution at the removal of Rod of Ages, but if the game calls for it you may need this item. I suggest this item would replace either Rod of Ages or Lichbane. Lichbane is good but the Rylai's Scepter slow will keep your opponant still while you attack and Lunar Rush will be back to jump if they try to run with Moonfall to prevent escapes. It is an overall reduction in damage output, if you are skilled enough to land your Crescent Strike your enemy will not escape you.

>>Mercury Treads- I consider this a legitimate alternative to Diana. What's scarier than a DPS? A DPS that cant be stopped. I suggest this in the games where Diana is targeted in teamfights. Another good choice would be ninja tabi.

>>Nashor's Tooth- Its a fantastic item. Really I am torn between using Malady and Nashor's Tooth. Both give stats that are useful to Diana. One reason I would prefer Malady is that it doesn't cost as much, yet still has potency. It may not have the bigger amount of AP, but the magic resist reduc debuff is well worth the money.

>>Hextech Gunblade- I worked hard to try and work this item in to my build. Sadly, it is not an easy task to accomplish. This item provides stats that are very useful to this Diana build. Since Diana uses the Unique passive of Rod of Ages for sustain till level 18, she has no influx of life or heal after that point. Which means that damage taken will most likely be permanent till Diana either dies, or goes b. The sustain from this item allows her to utilize her pure damage potential from both physical attack and from abilities. (I will note that while her passive does magic damage, it does not heal based on spell vamp! It requires life steal.) The pure damage potential through her abilities and her passive's area damage would ensure she would never stop gaining health. The largest problem with this item is that it costs a metric ****tonof gold. While, it is worth it to some degree, the money that would be spent earning it in the early game would be diverted from buying the pure damage that Diana requires to be a deadly assassin. If you happen to get fed, and the game goes into the late game, I recommend that you try to get this, ONLY AFTER you complete your original build.
(Note:With this build, many of the desirable items are very expensive. I suggest striking a balance between expensive and cheap items based on usefulness. Start by grabbing items that are expensive and usefulness will last, f.e. Rod of Ages, Rylai's Crystal Scepter, then switch to a cheap item that will give some bonus to a stat that you really need to buff at the moment, continue alternating so that your champ will receive items that are easy to earn and items that provide raw power. Once you reach your final build, you can start replacing items that were cheap with items that provide more offense, defense, or health, in the assumption that the game will continue.)

Build is a work in progress