Illaoi is a very easy to learn champion who can 1v5 many a people by just splitpushing toplane, I have played her for two months and have gotten the two easiest pentakills with her, here are the stats that you will get from the build:

Max Health: 5269.68
Attack Damage: 303.425 (With dragon 1)
Ability Power: 0
Health Regen: 92.4 per 5 Seconds
Mana: 980
Mana Regen: 20.3 per 5 Seconds
Armor: 149.036 (Without boots and with 5 enemies nearby)
Magic Resist: 140.344 (Without boots and with 5 enemies nearby)
Movement Speed: 488.25 (With fully stack Dead Man's Plate, boots, and Dragon 3)
Cooldown Reduction: 40%

+Lots of DMG early game
+All-Around good champion
+Hasn't been nerfed yet
+Easy to learn
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-None so far

Marks 1: One critical strike and eight flat attack damage runes, for squishy top laners.
Marks 2: Nine armor penetration, if you don't plan on building Black Cleaver.
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Seals 1: Nine scaling health if you plan to be a unstoppable tank with 5k health and extra dmg with your Titanic Hydra.
Seals 2: Nine flat armor if the enemy has heavy AD.
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Glyphs 1: Six scaling CD reduction and three scaling magic resist due to most top laners being ad and even then you could get a doran's shield for ap champs. CD Reduction gets you to 40%.
Glyphs 2: Nine scaling magic resist for heavy ap teams in other lanes.
Glyphs 3: Nine flat magic resist, FOR THAT RUMBLE CHAMPION HNNNNNNG!
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Quintessence 1: Three MS, for late/early game mobility, helps alot with chasing due to your w, it also makes the dash animation on it go faster.
Quintessence 2: Three flat AD, for extra early game damage and farming.

Cunning:
5 Savagery, helps with farming.
1 Runic Affinity (OR) Assassin, for Runic Affinity since the jungler is supposed to help you get rift herald to push your lane, use this. But if you plan on pushing for the whole game, get Assassin BECAUSE YOU WILL GET 1v5ed!
5 Merciless, helps getting those pentakills (or kills in general) easier.
1 Dangerous Game, THIS HAS SAVED ME SO MANY TIMES FROM DYING, and Spirit Visage improves it's healing factor as well.
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Resolve:
5 Unyielding, even though you only have three items with defensive stats other then health, it's still pretty useful.
1 Tough Skin, two less damage from attacks is at least something.
5 Veteran's Scars, you are gonna be building a lot of health and so this is gonna get you up to that 5k so that no one can kill you unless they 1v5.
1 Insight, Insight will help you if you get pushed out of lane (which you will if you are going against Renekton,) this lets you get that teleport a bit earlier or to help a lane with ganks.
5 Legendary Guardian, this will help in teamfights or those 1v5s to make you a bit more tanky.
1 Strength of the Ages, ah the crown jewel mastery, every time you kill a cannon minion you gank a bit more maximum health until 300 maximum. After that you get 100 health back per cannon kill, so 300 max health and some more lane sustain is pretty good.

Starting Items:
Doran's Blade, for really squishy champions that you can crush with little damage back.
Doran's Shield, for tanky AP champions.
Cloth Armor, most top laners do AD so this will dumb down their damage a lot.
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Core Items:
Titanic Hydra: AOE damage that scales with health, main item.
Sterak's Gage: Prevent you from getting nuked even with 5k health by giving you a shield.
The Black Cleaver: Armor Shred helps against tanky enemies, especially with all the tentacles.
Spirit Visage: Helps with MR/Health/Healing, really good.
Dead Man's Plate: Extra damage on full stacks with some extra movement speed and tankiness.
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Situational:
Ninja Tabi: Incase the enemy has more AD champs.
Mercury's Treads: Incase the enemy has more AP and CC.
Trinity Force: Chunks them if they don't have any tanks or off-tanks, use Armor pen runes.
Sunfire Cape: For champions who like to "hug" onto other champions for damage a lot.

We want to take the skill order or R>Q>E>W, R for obvious reasons, Q makes your tentacles from your passive and itself heal more after each skill point, after you get a Spirit Visage, THIS WILL HEAL FOR MASSIVE AMOUNTS. E is for if your enemy is stupid enough to stay in the bubble where he can get damaged by the ghost you took of him/her/it, each skill point makes it deal SIGNIFICANTLY more damage. W just makes the next auto attack that you use with this deal more damage so we take it last.

Flash: Take this always, you have no other engage or disengage unless you count your w that lets you lunge towards the enemy. Still you need a targetable enemy to do anything with it.
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Teleport: To help incase they are taking your tower or you are missing out on farm because they pushed you out of lane.
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Ignite: This is if you want to get first blood earlier and start snowballing, still I would recommend teleport instead of this.

This guide is mostly for my friends but thank you for reading it anyways.