Double-Edged Sword: 0/1 - It's a good mastery, but I just don't use it out of personal preference. Take a point of out fury if you do want to use it.
Fury: 3/4 - Fury is a good mastery for Rek'Sai as she benefits from AS as it lets her use her Q more often.
Sorcery: 4/4 - Sorcery is a must on Rek'Sai; She has a really long ult cooldown and with my Runes/Build, sorcery takes her CDR up to max.
Butcher: 1/1 - Pretty much an essential mastery on junglers, it makes you do more damage in the jungle, helping that early clear time.
Feast: 1/1 - A decent mastery when in the jungle or taxing a lane to restore some health and mana. If you don't like it, swap it for Double-Edged Sword.
Expose Weakness, Mental Force, Arcane Mastery, Spell/Blade Weaving, Archmage, Arcane Blade: These masteries are better off on AP Champions/AD Casters and thus are not necessary on Rek'Sai.
Frenzy: 0/1 - Unless you're running Crit'Sai, this isn't necessary.
Warlord: 0/3 - This mastery scales with bonus Attack Damage, and since we're not building full AD, these mastery points can be used elsewhere. for more benefit.

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. Every time you clear a jungle camp or move between lanes, use your burrow to sustain yourself. It's pointless to store up fury between ganks, it'll just deplete by the time you get to your destination.
All of her basic attacks and abilities except Burrow generate Fury.


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. This ability is your jungle clear/enemy damage/wave clear move. It also generates bonus fury. Activate it when in the jungle and while fighting, whenever available.

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. Since you're basically Lee Sin the Blind Monk while burrowed, this move helps you target your enemies while ganking and help you land your unburrows for maximum efficacy. It's also good for poking down jungle camps before you unburrow and knock them up. If an enemy gets away from you with low health during a gank,
a quick Q can easily pick up a kill.


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. This is seen as ripples on your screen every second or so, and can give you an idea of where enemies are when you're counterjungling or moving around the map.
Unfortunately, as a hidden Teemo is not moving, it's position won't be revealed.

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. This is your bread and butter initiation move. Chances are, you won't be able to use burrow twice in a gank, so make your positioning count and land it right the first time. The enemy should still be in vision range of your laner, otherwise they're way too far for an effective gank. Use this while clearing jungle camps to avoid
damage and do damage.


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. Use this when they're about to get away or to finish them off. Always try to use it at full fury, as true damage bypasses all defenses.

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. These are your main focus to make you as mobile as possible. You can chain together multiple tunnels for a jungle route that is fast and efficient as possible, or place them all across the map for anchor points for your ult.
Placing a tunnel in every lane, in each jungle quarter and one by your base is an effective strategy for ganking, and later in the game, backdooring. Effective tunnel placement is essential, as without it, your ult would only be a 20% AS buff.


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.
Nocturne Ult and Teleport had a lovechild and its name is Void Rush.

Defense:


Block: 2/2 - Makes champions do less damage. Makes your already squishy early game easier on you.
Tough Skin: 2/2 - Basically your go-to Defense option for junglers. It makes them do less damage.
Unyielding: 1/1 - Makes champions do even less damage.
Veteran Scars: 3/3 - With the jungle harder than before, you'll need as much HP as you can get so you don't die.
Juggernaut: 1/1 - Gives you even more health, with late game scaling. Wondrous.

Defense:


Block: 2/2 - Makes champions do less damage. Makes your already squishy early game easier on you.
Tough Skin: 2/2 - Basically your go-to Defense option for junglers. It makes them do less damage.
Unyielding: 1/1 - Makes champions do even less damage.
Veteran Scars: 3/3 - With the jungle harder than before, you'll need as much HP as you can get so you don't die.
Juggernaut: 1/1 - Gives you even more health, with late game scaling. Wondrous.

Items

Starting Items:

Hunter's Machete is essential for Junglers as it gives you sustain in the jungle, bonus gold per large monster kill and leeway for any of the smite upgrades.

Health Potions give you sustain in the jungle and between ganks, in addition to your passive.
Warding Totem grants vision around an area, letting you know where things are. You can use it to protect your jungle, or ward the river to give your laners some information about where the enemy jungler/roamers are.

Jungle Items:

Stalker's Blade for gank pressure/dueling power, Ranger's Trailblazer for beginners and jungle clear or Poacher's Knife for counterjungle Rek'Sai.
Skirmisher's Sabre isn't really necessary unless they have an invisibility champion.
Stalker's Blade gives you more CC (50% slow for 2 seconds, wow!) as your only CC is your Unburrow, which, chances are, you can't use twice in a gank. If they're starting to run, you can smite them and pick up the kill.
Stalker's and Ranger's are the most recommended smite upgrades to buy, with Devourer or Warrior being a must for your enchantment. Devourer gives you AS, on hit magic damage and scaling for your ganks. Warrior gives raw AD, Armor Penetration and CDR for your insane ult cooldown. Choose either based on preference.

Boots:

These are situational. You should pick up Tier 1 Boots of Speed whenever you can, preferably while building into Devourer or right after. Tier 2 Boots should be build at some point after your Jungle Enchantment. Usually, your boot upgrade will be Homeguard.
Mercury's Treads (Recommended for Most Situations): Its passive, Tenacity, reduces CC effects by 35%, so it helps you stick to better. Also useful for full AP teams.
Ninja Tabi (Full AD Enemy Team): If the enemy team is filled with AD Champions, e.g., Yasuo Mid, Darius Top, Any ADC, Vi Jungle, opt for Ninja Tabi. The 10% AA Damage reduction and 25 Armor really helps you stay alive. A fun rhyme to remember it by would be 4 AD, go for Tabi.
Boots of Mobility (Really Fed, Roaming): These boots make you go fast. If out of combat for 5 seconds, the 25 Movement Sped value is increased to +105. Only buy these if you're really fed and going lane to lane picking up kills left and right.

Offensive:

Ravenous Hydra: This item synergises really well with your Q, as well as giving you life steal to survive longer in fights. It gives you raw damage, health regen, life steal and a kickass passive/active.
It's core on Rek'Sai.
(Options for your Last Item)
Last Whisper: Take this when going up against tanks. Raw damage and its 35% Armor Penetration really helps. It's a good last-pick alternative to Guardian Angel.
Frozen Mallet: This item is sort of a cross between offensive and defense, giving AD and a ton of health. Its passive also helps you be super sticky and make stragglers unable to escape.
Black Cleaver: Theoretically, this item works because of CDR, AD, Armor Pen and its Armor Shredding Passive. I haven't tried this item out too often but I think it's a viable choice.
Youmuu's Ghostblade: Just like BC, this is theoretically a good item because it provides AD, Crit Chance, Armor Pen, CDR, and its active helps you stick to enemies better.
Maw of Malmortius: Maw of Malmortius is a good item if you're against an AP team but you still want to do damage to the enemy team. It gives damage, MR, more damage from its passive and a shield from its other passive.
Blade of the Ruined King: Not as good as Ravenous Hydra, but its on-hit damage and active help you stick to enemies longer. It gives you damage, attack speed, life steal and a passive/active that makes you do damage and stick to enemies. It's sort of like Youmuu's.

Defensive:


Spirit Visage: This item synergises very well with your passive, as it gives you increased healing. It gives MR, lots of health, CDR (Amazing), Health Regen and a kickass passive.
Randuin's Omen: This is my go-to armor item for fighters. It gives you a ton of armor and health, aswell as a passive that makes enemies attack slower and an active that makes enemies slower.
Sunfire Cape: You can use SFC as an alternative to Randuin's Omen. It gives good health and armor, with its passive synergising with your Q and Ravenous.
(Options for your Last Item)
Guardian Angel: When it's late game and you're really scared of that 75 second respawn time, GA is a great, safe last item pick as it gives you Armor, MR and a second life.
Banshee's Veil: Extra MR, Health and a Passive that negates the first ability that hits you and grants bonus health regen. Fantastic for blocking Nidalee spears, a general poke composition and even on the off chance that a Karthus ult comes around.
Thornmail: If the enemy is 4 AD or even 5 AD, just like with the Tabi, pick this item. It gives you 100 armor and a passive that reflects 30% of physical damage back to the attacker.
Warmog's Armor: Raw Health and a passive that gives you tons of regen? Count me in. I wouldn't pick it up that often, but if an extra life, poke blocker, damage reflection or Armor Shred don't appeal to you, go for Warmog's. It's not that great of an option because the passive isn't that great, and the raw health is basically useless without Armor/MR stats, but you can play around with it.

Elixirs:

Elixirs are tons of fun. Your three viable options are:
Elixir of Wrath: Gives you 25 AD and 10% of Physical Damage dealt is healed. Best for Normal Rek'Sai or Full AD Fed/Crit'Sai.
Elixir of Iron: Makes you 25% bigger, gives you 25% Slow Resistance and Tenacity, and makes you leave behind a trail that makes allies 15% faster. Best for Tank'Sai or Normal Rek'Sai, if circumstances dictate.
Elixir of Ruin: Gives you 250 health, 15% damage to turrets and makes allied minions do 15% more damage to towers and makes them gain movement speed based on your movement speed. Best for Normal Rek'Sai or Full AD Fed/Crit'Sai, especially if you took Boots of Mobility. This elixir is great for split pushing and makes Rek'Sai shine.


Jungle Routes

Read Haunt the Jungle for a more in-depth guide, or google it.

Your jungle consists of:
Red Brambleback Camp (Red Buff)
Blue Sentinel Camp (Blue Buff)
Raptors Camp
Wolves Camp
Krugs Camp
Gromp Camp

Rek'Sai's jungle route varies depending on how you like to play, but I'd narrow it down to 2 main routes.
Blue Side:
Route 1 (I prefer this one):
Rank up your Q and Get a leash at Krugs (Golems), meaning your laners will help you damage the monster, but back off when it gets low so you get all the EXP. Smite it at around 400 health. Rank up your W and use it to recover health. Go to the Red Brambleback camp (Red buff). Clear it, use potions as needed. Use your W again to regen. Go to the Raptor camp (A.K.A, Chickens), Clear it, and smite will come up when the Red Raptor is about
half health. Smite it, rank up your E. Set a tunnel for later purposes somewhere in that side of the jungle, and recall. Purchase your Smite Upgrade item and about 3 more health potions.
Go to your blue buff and clear it. Your smite won't be up yet. This is great if you take Stalker's, as it'll be up by the time you go and do your first gank.

Route 2 (S4 Traditional):
Get leashed at Red buff, smite it, clear the wolves camp (Wraith camp OP), then clear the Blue buff, smite it. Go in for a gank. There are some pros and cons to this route, it allows you to make sure your buff doesn't get counter jungled and lets you go in for an early gank. However, you don't get the smite upgrade and 10 bonus gold per camp.

Red Side:
Route 1 (Preferred):
Get leashed at the Gromp. Smite it. Take the blue buff, take the wolf camp. Recall, buy jungle item and 3 potions, go to red buff, go in for a gank. Essentially a mirror of Blue Side's route 1.