Note: Don't expect instant results, this takes lots of practice to perfect but when you get the hang of it you won't be disappointed. This guide isn't meant to be 100% serious for carrying ranked, it's just to have fun.
Already got a penta with this build in ranked :D

Pros
Good early game damage
Very mobile with ult and tunnels
Amazing scalings which result in really high bursts throughout the game
True damage E that has a broken scaling to it that allows it to get more true damage than smite
Has a movement detector that works in FoW
Burrowed Q reveals, resulting in no more face checking
Knock up sets up assassinations really well
Inbuilt sustain
High AoE damage

Cons
High Risk, High Reward
Countered by hard CC
Easily zoned early game by range champs
Can get kited easily
If behind E is useless
Can't do anything if enemy team sticks together
Vision restriction when burrowed is very dangerous

Well you gonna takebecause it is the best spell in my opinion.
Take for the kill potential as you will only be going for kills as an assassin and defensive spells won't help you there.

Starting Items:
x 3

This is my favorite starting set because the Long Sword gives nice damage and it builds into Brutalizer. The cost of Long Sword allows for 3 Health Potions which is very usual for lane sustain and trades.

First Back:


Try to stay in lane as long as possible to be able to buy a Brutalizer. If you have enough spare gold buy Boots of Speed. The ideal situation would be to get lvl 6 and have a Tunnel behind your tower, recall, buy these two items and some Health Potions or Wards, then Void Rush back to lane.

Core Items:


You should be aiming for these items through mid game.
Hydra is a MUST GET for Rek'Sai as the AoE synergizes so well with Queen's Wrath and the active serves for some extra burst that can be easily used during Queen's Wrath. Always build Tiamatfirst when getting Hydra. After this item build Last Whisper since basically all your damage is physical and the 35% Armor Pen from it will boost your burst a lot and will also allow you to hurt the tanks. Then finish of your Brutalizer into a Black Cleaver for even more AD, CDR, Armor Pen and some handy Health.

End Game Items :


For your 5th item get a Bloodthirster. The life steal is very useful and can be a life safer sometimes even the shield passive part but the main reason why we get this item is because of the AD. The 80 AD is useful because of course more damage but that means Furious Bite does an extra 160 TRUE DAMAGE (BROKEN). Get Triforce last because every single stat that it gives is beneficial for Rek'Sai. The Sheen proc is useful for burst and the Phage is good for sticking to targets and general mobility.


Situational Items:


I would only ever get Mercurial Scimitar if they had a hard CC ability that was the cause of me dying every fight because the active could be a life saver. Guardians Angel is good but its a pure defensive item (goes against my moral code) so only buy this if you are truly desperate. Only get one of these items instead of Triforce!


Boots :


Boots are an exception to my moral code. Get Merc Treads if you are versing an AP heavy team or they just have a lot of CC. If they have an extremely AD heavy team with people who AA for their main damage (Tryndamere, Yasuo, Master Yi, every ADC except for Corki) then buy Ninja Tabi. Only get Boots of Swiftness when you are super fed, snowballing hard and CC isn't a threat to you at the moment but you will eventually have to trade them out for either Merc Treads or Ninja Tabi late game.


Elixirs :


Just get Elixir of Wrath every time unless you aren't using Merc Treads and need some Tenacity then get Elixir of Iron. For split pushing always get Elixir of Ruin because the Tower Damage buff is very useful.

Marks:Flat AD - The AD scaling on his abilities are so insane that these are too efficient to not use.

Seals:Flat Armor/Flat Health - Take the armor seals if you are versing an AD heavy champ mid or majority of their team is AD. If the opposite happens use flat health seals for better trades.

Glyphs:Flat MR/Flat CDR - Same as seals, if your are versing AP mid or an AP heavy team use FLat MR otherwise use FLat CDR.

Quints:Flat AD - Basicly the same reason for Flat AD Marks.

Max Q first as this has some nice base damage early game and helps with trades, farming and generating that nice fury. The burrowed version also gets increased damage which allows for some more poke harass.

Max E second as it has no base damage, only scaling so this ability is not as good early game but its mid-late game is AMAZING!! The tunnel CDR is also handy.

Max W last because it gives you the least benefits and there isn't really any need for it other than the movement speed and the base damage. It is just more efficient to level this last.

Get points in your R when ever you can as the CDR is really worth it and the 20% attack speed each level is pretty nice.

Fury of the Xer'Sai (Passive): Activating Burrow with Fury will grant Fury of the Xer'Sai for 5 seconds, which causes Rek'Sai's next source of Fury generation to grant 15 bonus Fury and, so long as she remains burrowed, to expend 20% of her current Fury every second to restore health. The health restored is based on the amount of Fury expended, for a maximum of (25 × level) health over 5 seconds if she burrowed at 100 Fury. Fury of the Xer'Sai will not activate if Rek'Sai is at full health and her Fury will stop depleting if she reaches full health. This passive gives you some nice inbuilt sustain as it easily builds up when farming lane which allows for more healing to win lane.
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Queen's Wrath (Q - Unburrowed): Rek'Sai's next 3 basic attacks within 5 seconds deal bonus physical damage to her target and all surrounding enemies. When using this ability instead of generating 5 fury you generate 10 so always use it when possible to generate fury to end up using as sustain or getting that true damage on Furious Bite.
Prey Seeker (Q - Burrowed): Rek'Sai fires a pulse of void-charged earth forward in a line that damages the first enemy hit, dealing magic damage and revealing all enemies within (X)-range of the target for 2 seconds. When burrowed this will be used as a scouting ability too see what the ripples are. Since it will be doing hardly any damage don't bother using this in your burst rotation but instead as general safe long range poke or farm tool. After time you will start to see the damage count up.
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Burrow (W - Unburrowed): Rek'Sai burrows under te ground, reducing her sight radius to a small circle around her while gaining Tremor Sense, 15/20/25 flat MS based on level and an alternate set of basic abilities.

Tremor Sense: Enemy champions, minions and neutral monsters within 1300-range that move within the fog of war have their position revealed for Rek'Sai and her allies. This does not grant sight. Only use this part of being burrowed to scout the fog of war as the vision impairment is very significant and can lead to some 'WTF' moments where you don't actually know whats happening because all you see are ripples.
Unburrow (W - Burrowed): Rek'Sai bursts out of the ground, dealing physical damage and knocking up surrounding enemies for up to 1 second, based on their proximity to Rek'Sai. Enemies cannot be affected again by Unburrow for 10 seconds. Using a basic attack while burrowed triggers Unburrow. You should always try to initiate with this move as it will cc them which helps you burst them without them being able to react and you gain bonus fury when you next do an ability which means more damage on Furious Bite. This can also be used when ever farming just to get that aoe clear even better.
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Furious Bite (E - Unburrowed): Rek'Sai bites her target, dealing physical damage that is increased by 1% for every 1% of her current fury. At full fury, it deals true damage.
This move starts at 80% of her AD and 160% at full fury, and goes all the way to 120% AD and 240% AD at full fury. This ability is broken in my opinion as the scaling allows you to actually contest objectives because it does that much true damage. The maximum amount of damage I have ever dealt with this is 1200~1300 true damage. THAT IS MORE THAN SMITE!!. Try to use this as a kill secure or the last part of your burst as the more fury you generate, the more damage. Always try aim for that true damage mark and don't be afraid at lvl 2 just to walk up to your lane opponent and just bite them for true damage. IT HURTS.
Tunnel (E - Burrowed): Rek'Sai creates a tunnel and digs toward the other side, connecting two tunnel entrances. Clicking on either of the tunnel entraces will take you to the other side, activating a 12 second static cooldown. They last up to 10 minutes and Rek'Sai can have up to 8 pairs on the map at the same time. Enemy champions can destroy tunnel pairs by standing on either entrance for 1.5 seconds. However, if Rek'Sai is moving to the other side via Ult or by interaction, it will not stop her. This is an amazing tool as it can be used as a gap closer or an escape, even a teleport point for your ult. Try use these in lane to bait your opponent and then go through them when they are trying to destroy it and get an easy trade maybe a kill if the opportunity arises. Put these tunnels at crucial points and always have one behind your turret while laning to use a an ult spot just to get to lane quicker as you probably won't be using it for anything else that early in the game.
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Void Rush (R): Rek'Sai passively gains 20% bonus AS per rank in it.
Targeting any of your Tunnel entrances will cause you to channel for 1.5 seconds and rush to that entrance if you do not take any damage from champions or turrets. If her channel is interruped, it will be placed on a 1.5 second cooldown.

Think this as a global Nocturne ult but just to you tunnels. The passive attack speed is a nice addition to your general dps.

Basic Skill Rotation->-> -> -> ->
Tunnel and make your way towards your target and shoot you Prey Seeker at them, next part is to knock them up with Unburrow. If you can't get in range then try use one of your existing tunnels or just make another one as a gap close. Once they are knocked up use Queen's Wrath to deal insane damage, this is like Xin Zhao's Q but instead it gives more AoE damage whilst applying that damage on top of your AA. Try pop your Tiamat or Hydra while doing so and keep AA them until they get away or they die but try Furious Bite as late as possible. Act as if it's a kill secure ability like some greedy Lee Sin's do with their ult's, but the longer you postpone it the more damage it will do (just think of the true damage :D).

Let's do some math and see how much damage the Basic Skill Rotation without anything extra (more AA) will do with just the 4 first items: Ravenous Hydra, Last Whisper, Black Cleaver and Bloodthirster while being level 18.

113 Base AD
75 AD (Hydra) + 40 AD (Last Whisper) + 50 AD (Black Cleaver + 80 AD (Bloodthirster) = 245 Bonus AD + 3.5% (Masteries) = 253.575 ~ 254 Bonus AD
254 Bonus AD + 113 Base AD = 367 Total AD

Prey Seeker = 180 (+1.0 AP) = 180 Magic Damage
Unburrow = 260 (+0.5 Bonus AD) = 260 + (254/2=127) = 387 Physical Damage
Queens Wrath = 95 (+0.4 Bonus AD) x 3 = 95 + (254x0.4=102) x 3 = 197 + 367 (Total AD (Basic Attack)) x 3 = 564 x 3 = 1692 Physical Damage
Hydra Active = 100% Total AD = 367 Physical Damage
Furious Bite = 240% Total AD = 881 True Damage

Total Damage dealt with just abilities and no resists:
180 Magic Damage
387 + 1692 + 367 = 2446 Physical Damage
881 True Damage
180 + 2446 + 881 = 3507 Total Damage

Now lets take into account the armor and mr, for example Caitlyn at lvl 18 with flat armor and mr runes.
Base Armor = 82.4 + 9 (Runes) = 91.4 - 65% (Armor Pen 40% + 25% from Black Cleaver) - 10 (Flat Armor Pen) = 22 effective Armor
Physical Damage Reduction = 100/100+22 = 12% = 2446 - 12% = 2152 Physical Damage
Base MR = 30 + 11 (Runes) = 41 effective MR
Magic Damage Reduction = 100/100+41 = 21% = 180 - 21% = 142 Magic Damage
Total Damage After Reduction = 2152 + 142 + 881 = 3175

3175 IS A LOT OF DAMAGE!!!!!!

Level 1 - Versing Melee :
This is easy because with Queen's Wrath you can easily out trade them. When they go to last hit just walk up to them and Queen's Wrath them and walk away. Trade won.

Level 1 - Versing Range : Don't even try attacking your opponent at level 1 just focus on dodging all their harass. If the opportunity arises when their only spell in on CD and they are out of position then go in for some Queen's Wrath then run away. They should be squishy as ranged. Try to push the wave and get level 2 first. So they have to toss up either poking you or last hitting under tower.

Level 2 :
Same this as Level 1 but you should always be using Queen's Wrath when ever it's not on CD as it generates fury faster and does more damage. Try get 100 Fury and go up to you lane opponent and Furious Bite them and walk of. Easy ~130 true damage for free, if you can get some Queen's Wrath in go for it as this will make your trade even better. Keep trying to dodge poke and find opportunities to harass them with Queen's Wrath and Furious Bite.

Level 3-6 :
This is the best part as you know have your full combo which can easily lead to a first blood if played well. Your going to want to Burrow and poke with Prey Seeker when ever it's not on CD. Wait for you opponent to use some of their abilities to harass or farm and when they are on CD Tunnel to them and Unburrow and start your basic rotation. You can always just stop mid rotation to back off if it will be more benificial for the trade.

Early game have a tunnel somewhere near mid lane to Void Rush to just for utility purposes. It's like an inbuilt teleport early game.

At this point you should have/
You should have high burst damage with Queen's Wrath and Furious Bite should start to scale now. If you haven't started going for kills you better start now. Roam bot/top lane if you see kill potential and don't underestimate your damage. Using your basic rotation on the ADC will result in an easy 100-0 (ignite if needed). When roaming to other lanes make your you travel Burrowed because you can scout their jungler to be safe and you also get some extra movement speed.

When you get to the lane try save your Tunnel for when they use their escape like flash so you can stay close to them. Use your Prey Seeker to get vision (you can trinket ward the lane if you desperately need vision), you will be Burrowed for the majority of the time because you need Unburrow to set up your Rotation. Focus the squishies and remember. YOU ARE AN ASSASSIN NOT A TANK, YOU WILL DIE QUICKLY IF FOCUSED!

Try keep Tunnels around the map in key locations so if the situation occurs where you need to be there you can just Void Rush there.

At this point you should have near full build and around 400 AD. Your damage will be so high that you can literally 1v1 anyone but don't get too cocky as your are still squishy and can die if fighting more than a 1v1. Keep in mind that their ADC and mid laner should die in 1-2 seconds if you can manage to get your basic rotation on them.

You can also help contest objectives as Furious Bite will be doing around he same amount of true damage and sometimes even more than smite (BROKEN).

Both teams should be roaming around together at this moment so stay in the jungle or flanking position and make sure your in not in the team fight when it starts. Just wait for everyone to use their initial moves and then Tunnel to their carries (if they are still alive) and do you basic rotation which is a guaranteed kill. If they are already dead just help clean up as you damage is INSANE at this point in the game.

You can also split push if it is the best option because your kit is pretty good for it. Since you are building full damage the 60% attack speed for Void Rush helps a lot and those turrets get destroyed really fast. Burrow to keep 'vision' of the FoW and keep Tunnels around for escapes. If one person comes to try stop you, just kill them because no one (that I have versed yet) can beat me in a 1v1 when I get the first blow with Unburrow.

Note: Furious Bite has a 2.4 Full AD scaling at lvl 5 when full fury so when you get the maximum amount of AD with this build and all the other buffs which is around 500 AD. Furious bite with full fury will deal 1200~1300 true damage. OP!!

Thanks for reading my Rek'Sai guide :)

I hope that this guide has taught you the ways of how I play Rek'Sai and see the potential he has as an assassin.

Feel free to leave a comment and any feedback is well appreciated!

If you want to ask me questions personally just add me on OCE server, my name is '1337 Noble'. (Sorry if you can't get on the OCE server)