1. Anyone is welcome to change in the build and try new runes/items combination. the above items/runes are my personal pick after many experiments.
2. Anyone who has a suggestion of a change or a build can put it in the comments, I will add it after either you get some upvotes by people who tried it and said it's useful or showed evidence of it working (videos or photos) or i try it.

Why lethality caitlyn:For those of you who remember from previous seasons Lethality Caitlyn was one of the most feared champions, only few dared to try it on ranked, it was very risky but highly rewarding on success.
Why try it:This season it's even harder since you need 5 items for 100% Critical strike, however you only need 2 for 58 lethality (100% damage on non-tank) which made me want to try it even more and I had a lot of success late game but it was real hard early since it depended highly on the support.
Why make the guide:The thing that pushed me to make this is that I see so much potential in this build, as you will see you can easily two shot enemies late game but no one seems to take the risk, plus I want to hear your suggestions on a better early and possibly late game builds.
What is this guide: In this guide I'm going to explain and show you the results of my experimenting up until this point so you won't start from 0. i really want to perfect it.

Pros

+ One of the best late game champions
+ Good whether in front or behind
+ Insane Damage late game, trapped enemies are dead enemies
+ Enemy lifesteal almost useless (dead before they can attack)
+ Very rewarding when mastered
+ Great poke
+ Can be great for siege or defending turret (placing 5 traps in front of turret/base entrance) enemies wont dare to get close.

Cons

- very hard learning curve, need time to learn where to place traps
- not very good early game
- can be vulnerable to enemy ganks and assassins
- non-headshot AA aren't as good

Main tree
Hail of Blades is the one I had most success in, your full AD/Lethality makes your AA hit like a truck, three of them in a row can assure you a takedown, very useful from the start and starts working instantly.
Dark Harvest- increases Damage (scale), especially useful when sniping enemies with Ace in the Hole.
Fleet Footwork- gives you some nice sustain and the movement speed lets you land a second hit on poke or quick escape.
Hail of Blades can be very useful since you don't have a lot of attack speed but your damage is high when getting ganked this rune lets you deal massive damage in short duration. and even kill

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- Cheap Shot goes well for Yordle Snap Trap(bonus true damage) and Sudden Impact is better for 90 Caliber Net and its bonus lethality continues for the rest of Hail of Blades attacks. choose depending on enemy team. since you have good healing from Bloodthirster (or Vampiric Scepter early game) Taste of Blood is useless.
- Presence of Mind is my go to rune since it gives you good sustain, Triumph is also viable but the close calls when it's useful are not much present since you keep your distance. Overheal useless since you have Bloodthirster (maybe useful early game)

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- Eyeball Collection or any other are good since they give the same stats.
- Legend: Alacrity because since you are full AD this 18% makes huge difference, Legend: Tenacity is ok against heavy cc team but you keep distance anyway, Legend: Bloodline is inferior to Legend: Alacrity because on fights the bonus damage of Legend: Alacrity gives almost as much healing as Legend: Bloodline and deals more damage

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- Relentless Hunter is my go to rune since it gives you out of combat movement speed equal to Relentless Hunter which reduces the downside of not having boots. ( Ultimate Hunter is also good because it lets you use your ult more which will grant you (hopefully) more kills however late game it only takes off obout 5 seconds which is not a lot, Ravenous Hunter is ok but but i believe that Bloodthirster gives you enough healing for sustain, and since fights end in 2-3 shots late game it's not as useful.
- Coup de Grace is my pick for this one since get gives you 8% extra damage for low health enemies which translates to 100+ damage to enemies with Ace in the Hole and headshots late game. Cut Down is also good when facing tanks. Last Stand isn't as good as the other 2 since you should stay high on health

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Secondary tree
- Gathering Storm is more or less obvious since your headshots late game can triple this amount.
- Absolute Focus is the more viable option to me since I said before headshots triples this amount late game 18*3=54 bonus damage (full is dealt since high lethality), since you will be keeping distance you'll have this on when poking and damaging trapped enemies. Transcendence is ok letting you reduce cooldown but not as useful.

- Infinity Edge, The Collector, Bloodthirster are obvious choices
- Essence Reaver is one of the most underrated items, not only does it give you some mana for sustain, its 40%+ bonus AD after using an ability increases your damage insanely, you will be using it so much after using Yordle Snap Trap under rooted enemies or after using 90 Caliber Net.
- Lord Dominik's Regards is really useful if enemy has tanks.
- Navori Flickerblade is also a highly underrated item, not only it has a lot of AD, it also reduces cooldown on abilities on critical strike, means you can reuse 90 Caliber Net after a short time in case the enemy isn't dead.
- Berserker's Greaves gives you some speed for escaping ganks, (essential on laning phase) and some attack speed. late game you won't need it since you will be a moving turret, teammates should protect you from assassins and you have Hail of Blades for taking them down and 90 Caliber Net for dodge.
- Galeforce gives you AD,Critical chance and attack speed, all you need on laning phase, plus the most important one a dodge option.
- Rapid Firecannon only useful for poking.
- Sanguine Blade only useful for mid and it's the best Caitlyn laning phase item. lifesteal, damage, lethality, attack speed.
- Prowler's Claw is one of the most underrated items on caitlyn, its mythic bonus gives you up to 46 lethality (21 from item itself) combined with The Collector can give you 58 lethality negating all non tank enemies' armor making your attacks/ abilities deal 100% damage.