Diana is a pretty strong unit in TFT and can be made into hypercarry fairly easily - especially if you hit the “Chosen” buff on her. On top of having a good trait in Assassin, Diana also sports Moonlight, which can be used to make you much stronger than you actually are at any point in the game. Plus who doesn’t love hitting Moonlight on 3-star units?
When should you go for this comp? This composition can and will work without “Chosen” Diana, but it will be harder to pull off. If you do hit a “Chosen” Diana, it’s not even a question on whether or not you should build this. Without her, you'll need to make another unit a Chosen Moonlight or sub in a Moonlight for another unit.
Because Diana is a 1-cost unit, we’re going to employ some slow rolling to give ourselves better odds of hitting a “Chosen” version of her. Your goal for all of Stage 2 (i.e. before Krugs) is to not level with gold and save as much gold as you can. You’ll of course want to spend them on Dianas if they appear.
On top of Diana, you’re also going to be looking for Lissandra, so if you happen to see those while you’re waiting on Krugs, grab those as well. However, any Moonlight or Assassin units are also good buys as you can begin activating Diana’s synergies to make yourself a bit stronger.
After you’ve passively hit level 5, it’s time to start slow rolling for Diana and Lissandra. Again, we’re ideally trying to hit a “Chosen” buffed Diana, but as long as you’re accumulating Dianas and Lissandras you’ll be in an okay spot.
Similar to what you were doing in the early game, all you need to be doing in terms of the rest of your board is adding more Assassins to buff Diana and the Assassin trait further. Units like Katarina, Akali, and Talon are all excellent choices.
The game plan changes post-wolves depending on your game.
If you’re strong or doing well enough that you’re not afraid of dying - You’ll continue slow leveling through levels 6, 7, and 8 to add more units and upgrade your existing ones. Moonlight and Assassin are the two things you want to prioritize when you’re hitting the most expensive units, as they are the two most important buffs for Diana.
If you’re feeling weak or scared of dying, you’ll need to take stock of your lobby. If pivoting is your best option, feel free to do so. If you’ve hit a decent amount of Dianas and Lissandras and/or have good items for Diana to carry then you can stick with the main plan. Although either hyperrolling to finish finding copies of your main two units or adding trait buffs is a better use of your gold than leveling if your units themselves are still fairly weak. If your units have been tiered up, then you may want to consider leveling fast to add higher cost units to your board that may be able to help you stabilize easier.
Late game usually unlocks the best units, the 5-cost units, which are always great to use at some capacity. More on those next.
Supporting Units (Non-Moonlight or Assassins)
There are basically three high cost units that I would recommend adding to this composition.
Morgana: Obviously a strong unit individually and a good kit for controlling/winning fights. She’s an excellent pickup because she’ll get Lissandra's Dazzler trait to activate and grant those two attack damage reduction to their targets. Great for stomping out heavy auto attack focused compositions.
Azir: Azir really just checks a lot of boxes for a lot of compositions with his Emperor trait. His sand soldiers really allow you to manipulate and protect your board in ways that you couldn’t with other units. Preventing enemies from jumping to your backline or disrupting the march of the enemy frontline is just too valuable to be put simply.
Kayn: Kayn being able to transform into either a strong frontliner or strong damage unit gives you some much needed flexibility in the late game. Because you’re focusing on getting your Diana hooked up with items first, Kayn can fill multiple roles based on your item scraps or lack thereof.
Aphelios and Sylas are for those game where you have a great Diana set up, but she's not "Chosen". Running either is fine as their only there to complete the Moonlight buff. Outside of that, they're just fodder.
Your main goal is turning Diana into a hypercarry, so the items I’d suggest are:
Hand of Justice: A nice increase to damage on either side to let her crush the enemy quicker.
Infinity Edge: More crit chance and damage means one shotting enemy backlines and that means victory
Runaan's Hurricane: Diana’s going to be smacking the enemy units a lot, Hurricane lets her hit multiple units at once. What more could you ask for?
Giving Lissandra a Zz’Rot portal or any form of supportive/mana gain items are solid choices, should you be able to afford that luxury.
Morgana can be a unit that you put extra damage or more supporting items onto, either option will turn out well.
If you can find a Hextech Gunblade, seriously consider giving it to Kayn and having him transform into his Blue form. He’ll be a nice boost to your damage. If you don’t have one to spare, going Red Kayn is the safer/better option.