This build takes advantage of the 3 strikes on Q, the insane AD scalings in her kit, and the True Damage on her full fury E. At full build, E with full fury deals close to 700 True Damage since it scales with 170% Bonus AD. This also combines with the high amounts of armor pen gained. In all, this build is going to be about going in and deleting the carries, then following up by shredding anyone nearby with your overwhelming power. I'll be going into more detail involving the rest of the guide below.

Insane Damage Output: Rek'Sai already does high damage, but with this builds combination of crit and lethality, you can shred most champions that aren't just EXTREME tanks like Full Build Ornn or 15 stack Cho Gath.

Good Farming: Her Q makes farming relatively quick and easy, especially late game when you Q and take the entire wave. It'll be very helpful for keeping up on gold and cs through mid and late game.

Great Teamfighting: Something that's a bit unique about Rek Sai as an assassin is she can get to an enemy carry by hitting them from afar and then getting a free Teleport, where as most assassins have to get in close first and THEN R to use it effectively and aren't gauranteed to hit unlike Rek Sai(with a few exceptions like Shaco R and Yi Q timing) This unique part of her kit makes her difficult to ignore in a fight as she can destroy backlines out of nowhere.

Extremely Squishy: One downside of her in general is that she's extremely squishy and like most assassins, will want to avoid hard CC at all costs. It's one of her few downsides past lane phase.

Coinflip Early Game: Early Game isn't bad, but it can sometimes feel like a coinflip considering a lot of the top meta picks can roll her and make the lane feel impossible. Similar To Gwen, Rek Sai excels at level 1 and 2 all ins, then falls off until she gets level 6 and can start getting close without taking a bunch of poke first.

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  Hail Of Blades works as a keystone for Rek Sai because it interacts with her Q, meaning we gain the attack speed and can get some incredibly quick Qs in the early game. This interaction takes many people by surprise in level 1 and 2 trades and makes it so you can Q+E+R faster.
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  Cheap Shot is good because you can knockup your enemy then get cheap shot for the next combo, which is almost always going to be free damage.
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  Eyeball is going to be for the bonus AD we get, and because the other two runes in the same branch are too situational to consider over Eyeball.
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  Ravenous Hunter has been nerfed a bit in recent patches, but I still think it can work on Rek Sai due to how aggressive our late game is going to be.
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  Triumph is going to give some minor gold advantage and restore health on takedowns as we dont need Presence Of Mind and we don't have enough healing to consider Overheal.
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  Cut Down is going to fit nicely into this build as with max build we only have about 2000 health, meaning bruisers and tanks are going to feel it the most. This rune will make it so you can be just as effective at killing bruisers as you are at killing squishies.

Kraken Slayer is especially good with Rek Sai because the 3 hit passive aligns with her Q and you get the true damage even when 3rd hitting with Q. By the time you hit full build you'll be dealing 188 additional True Damage from kraken slayer. That damage paired with Rage E means you'll be dealing close to 900 true damage in a single Q+E combo, plus the additional damage from the other 2 Q procs. Since her Q damage got buffed recently, Kraken slayer will benefit even more. You can win early trades, but it'll be a lot easier once you get Kraken Slayer as most champions wont have enough hp stacked up to counter your True Damage output.

Berserker's Greaves are important as 2nd item because you always want to be fast and be able to close the gap between you and an enemy as soon as possible. The attack speed gained from greaves also helps with trades and assists kraken slayer mythic bonus to ensure we won't need to worry about hitting people too slow.

The Collector Is an amazing item for Rek Sai because it gives her all the stats she wants. Crit, AD, and Lethality. Add on the gold gained from passive and this is an excellent early to mid game item to get your build rolling in faster and to create openings to crush anyone that isn't an armor loaded brick wall. Having early crit is another major pro of this item because your Q can crit on all 3 procs. These Q crits can turn a fight into a massacre that would make Vlad The Impaler wince.

Lord Dominik's Regards is going to be best for the armor pen, and with taking out tanks and bruisers, as you are especially squishy with this build. It also gives you another 20% crit chance which brings the total up to 60%. This item is going to give you your power spike into late game to avoid falling off the face of the earth. At this point squishies and bruisers alike die on full combo and will chunk the ever loving **** out of tanks that aren't mega healing meatheads like Dr. Mundo.

Axiom Arc is a great assassin item in general, and that's no different for Rek'Sai. She takes advantage of the R reduction extremely well and with how game changing her R is in a teamfight, allowing her to completely ignore front line or become untargetable to avoid annihilation and either destroy a carry or force the enemy team to pay attention to you while your team shanks them in the back. On repeat her R is pure cheese, so we're going to run Axiom Arc to age that cheddar.

Infinity Edge is an amazing item for Rek Sai as your Qs can crit and we're going crit anyway so might as well go heavy on the crits. This item also gives the most base AD on a legendary so it puts a good amount of power into your maxed E's true damage. All around this item is just really good and worth the purchase.

For this build you'll be starting with Dorans Blade, and depending on the matchup, you can very well win a level 1 or 2 all in with the True Damage on E and Hail of Blades. You're squishy as all hell early so unless you're ahead, it's best if you aren't overly aggressive or shoving up too often. One helpful part of Rek Sai's kit is her passive. If she has fury as she enters the ground, she consumes the fury for HP over time. The passive adds some sustain and survivability in lane. Your first buy will be Noonquiver for the extra CS damage, and attack speed which stacks on Rek Sai even more

Mid game is the beginning of the end for the enemy. By now you should have Kraken Slayer, boots, and Collector. At this point you've already gained 40% crit, and 12 lethality. This balance of stats gained makes popping enemy laners easy, and can even take out bruisers with ease. The important part of mid game is going to be staying alive and keeping up on gold and CS. Kills are also important, but late game is when the build is strongest, so you'll have plenty of time for kills.

By Late Game you should be mostly full build, barring some exceptions involving Ghostblade. At this point, you've gained 80% crit chance, 400+ AD, and you now deal close to 700 True Damage on full fury E. Aside from armor focused bruisers, and full tanks you will delete any enemy with something as simple as a Q+E combo. If you can split up enemies, getting picks and even 2v1ing is a possibility. This build is incredibly powerful and volatile. Late Game is the true test of that statement.

In Conclusion, this build is a nuclear rocket, clawing people to death at mach 5. your stats gained from the build are potent and varied. Most lane Rek Sai builds run full lethality, but what they're missing out on is the crit potential on Rek Sai Q, and the added True Damage from every 3rd Kraken Slayer hit. Hail Of Blades is the perfect rune for this build. I hope you enjoy this build as much as I do, GLHF!