Hey
I played a lot of Rek'Sai recently and want to write a short and clear guide about the best runes, masteries and items. Its all focused on jungling, but much overlaps with laning.

Its all my opinion and there is no 100% best item build for everyone, in every case, so i put in options.
If you argue with some choices or have additional offers, let me know so i can learn from you =)

• You want some AD for damage early on
• You definitely want Armor for more sustain in the jungle
• CDR is nice to have but not necessary -> go for 9xMR, if you want
• 3xAS / 3xAD -> matter of taste

Go for 18-0-12 or 18-12-0, if you want to go more aggressive.

1. Upgrade your Hunter's Machete into a Stalker's Blade first.
   • Enchant it with Stalker's Blade - Cinderhulk as soon as possible to get more sustain for ganks.

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2. Buy Boots
   • you could instantly upgrade them into Mercury's Treads for great mobility and map presence, but you can push back this if you want to have damage earlier.
   • Even Mercury's Treads - Homeguard in combination with Void Rush is really nice to have, even early on.
   • Always go for Mercury's Treads, unless the opposite team has no CC/AP
     -> Go for Ninja Tabi then.

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3. Go for one damage item now, as you lack in damage at this point.
   • Titanic Hydra is a great choice, because of the AoE damage scaling with your health, the additional use effect and of course the bonus health.
   • Black Cleaver is also very nice, especially against champs who stack armor, but you also stick on your target because of the second passive and you gain more mobility through 20% CDR in synergy with the Tunnel of Furious Bite / Tunnel and Void Rush and again the bonus health.

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4. Now that you have a damage item and the passive of Void Rush, continue the tank build with two armor or magic resistance items, depending on your enemies.
   • If you need MR, go for Spirit Visage to amplify your Fury of the Xer'Sai or buy Banshee's Veil to get the spell shield.
   • If you lack in AR, you can decide to go for Dead Man's Plate for even more mobility and a powerful first hit or take Randuin's Omen for less incoming damage and more stickiness.

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5. You should have the jungle item, any kind of boots, one item for damage, one for MR and one for AR (or two of a kind if there is a full AD/AP team) by now.
   • You have to decide, if you are tanky enough to go for a second damage item, or if you lack in AR or MR - i put many items in the enumeration that could fit, but some items are less likely to be eligible.

• With the passive of Void Rush you should have enough damage with only one damage item, so focus on your tank stats.
  
• Your greatest advantage as rek'sai is your outrageous mobility and map presence.
  so spread your Furious Bite / Tunnel all over the map to get engage/escape routes and rush points for Void Rush to perform gank/rescue operations or simply save time.
  get CDR to have both abilities ready as often as possible and get movement speed to get faster from tunnel to tunnel. ;)
  
• Furious Bite / Tunnel is also a great tool to initiate ganks out of the jungle (and of course also teamfights later on).
  your damage isn't that crucial early on, so count on the surprise and try to buy time for your teammates to do the damage by knocking up the enemies with Burrow / Unburrow.
  
• A big downside on her is the missing stickiness/gap closer with your Furious Bite / Tunnel on cooldown after engage.
  Stalker's Blade helps a litte and the passive of Black Cleaver also performs well.
  Frozen Mallet would also be great in this situation but there are other more useful items in most cases.
  you can also think about flashing under the enemy while burrowed and unburrow immediately to perform a knockup and secure the kill (pay attention to the immunity cooldown).
  
• Use your Furious Bite / Tunnel to get into/out of the Dragon or Baron Nashor pit to do them sneaky or steal them.
  
• In teamfights, try to tunnel to the backline, perform a knockup and maybe get a quick kill as well.