Pro
-High Damage
-Good Knock ups
-Enjoyable Ganks
-Mobil asf
-Tunnels
-GRRRRRAAAAWWWWW

Cons
-If you fall behind you are F**ked
-Tunnel Reliant
-Slow

When you have the tunnel ability active USE IT!
the more tunnels the more places you can ult too and the more tunnels the easier it is for you to run if you get caught out.

Use Rek'Sai's tunnels to go in! Wait for the enemy champion to over step tunnel in knock them up then destroy your q key, if they flash use your chilling smite and tunnels to catch up with them.
If you get out played leave again using the tunnel you came in with. Easy Peasey

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.