Build Path: Nashor's -> Boots -> Mythic -> Lich Bane -> Crystal Scepter -> Cosmic Drive
This build is entirely centered around Night Harvester. The goal is that you have a little bit of added MS from your runes and items allowing you to get your Q onto your target before they can position away. Once you have marked a target with your Q Night Harvester will prock, giving you a 1.5 second window to close the distance for your dash-ult combo. Very basic principle that exectues perfectly with your Electrocute. Q proc -> Dash -> AA -> animation cancel with Ult -> AA -> Shield -> AA with passive proc. Then AA and use shield reset or dahs reset to finish the target off as needed. You have so much damage in this kit already, the Night Harvester just enables it to work flawlesly.
Nashor's Tooth: You will actually build this item first most of the time to improve your clear time and objective taking potential. You could theoretically rush mythic first if you got enough early kills but I just don't see that as being viable in the jungle without the proper attack speed buff from Nashor's.
Boots: Magic Pen boots are your go-to option but if you feel you need the CC resist or the AA resist build Steelcaps or Merc Treads. Mobility boots are a possiblity as well if you want maximum vision and objective control but I don't recommend them because they are a much weaker dueling item.
Mythic: Build Night Harvestor for the reason's listed above or build Chemtank with Predator for the insane gank potential. It really is up to you but unless you have experience with predator builds you will most likely get much more value out of Night Harvestor.
Lich Bane: This item is a no-brainer must-build item. The Sheen passive is just insane for all of your ability -> AA -> ability -> AA combos. The item as a whole increases your burst potential and gives a little bit of extra MS which adds to your presence as an initiate for your team. You could build Shadowflame here instead if the enemy team has some shielding or is very slippery and you cant reliably hit your auto attack combos, but in general with blu smite and night harvester you should be able to stick on your targets and that makes Lich Bane generally superior. But feel free to swap them out or even build them both should you want to maximize your magic burst.
Crytsal Sceptor: Just an overall good pick up for Diana. The added health makes you just a little bit tankier and the slow on your abilitites maximizes your sticking power. This will enable your teammates to really help you run down those enemy carries that are harder to lock on to. This item is a "nice to have" though and can be replaced by a matchup-dependent item like Banshee's Veil or Zhonyas should you need the extra defense.
Cosmic Drive: Your last item is just basically a last minute buff to the principles of the kit you have been building. You get more movement speed, more ability haste, and more burst potential. The ability haste is actually more important than the MS here which is why there aren;t any good substitutes for this item. At this point in the game you will be max build and fights will be longer, so you will desperately want to be able to get your combos up as fast as possible. While Diana does largely rely on cooldown resets to increase her dueling potential, in those late game fights where you are getting CC'd all the time this extra ability haste will save your life and allow you to keep on fighting. This is a situational "nice-to-have" as well, so you can go ahead and replace it with more comp-dependant items as needed. But if you are ahead or you don't really need any extra survivability then Cosmic Drive will be your best friend in the late game.
Chemtank Expanded: This item is so fantastic on Diana and is one of the most slept on items. After building Nashor's first (because you need it), Chemtank will enable the fastest ganks possible for this champ, and give you extra pressure on objectives. This is really good against junglers like Shaco or Nunu that like to sneak Rift Herald and Dragon when their teammates are still in their lanes. Boost yourself into the fight with the Chemtank and Predator active and watch as their health drops from your aggressive combos. The mythic passive will also make you just a tiny bit tankier (which helps those aggressive inititates0 and will further increase your ability haste. Not as good for 1-on-1 dueling as Night Harvester, but it is an incredible off-meta pick that gives you an unrivaled initiate and overwhelming team-fight presence.
