Hello,
I am LeifDaBeef, a Support and Jungle main and this is my guide to Rek'Sai. I wanted to make this guide because I see many Rek'Sai players not playing her correctly so I need to make it clear!!!

Marks: I bring Greater Marks of Attack Damage to do more damage overall to creeps and enemies
Seals: I bring Greater Seals of Armor so I can survive the jungle early game
Glyphs: I bring Greater Glyphs of Magic Resist so I can have more tankiness overall.
Quints: I bring Greater Quints of Attack Speed so I can attack faster and get more energy via the jungle.

I bring STRENGTH OF THE AGES so I have an easier time farming. If you plan on dueling, do not be afraid to use GRASP OF THE UNDYING instead. It is also useful if against a Shaco as Shaco players often invade at level 2.

I like to run a bruiser build on Rek'Sai that has good damage and quite good health and tankiness. It is best to get WARRIOR ENCHANT and then rush SUNFIRE CAPE for early game power.

STALKERS BLADE-WARRIOR Makes Rek'Sai's damage stupid early game, and you can duel pretty much anyone except Elise and Udyr.

Getting BLACK CLEAVER or TITANIC HYDRA right after CINDERHULK is not a bad idea either.

It is best to max first due to the damage and energy you get, then it is best to max because the knockup lasts longer.

Flash is a must have on Rek'sai so you can make escapes. It is also fun to try tunnel then flash jukes.

Smite is needed as jungler no matter what for dragon, creeps, and buff steals.

Rek'Sai is a jungler about map control and presence with her tunnels and the ability to instantly go to any one of them and punishes people for getting very kill greedy as she will come in, knock them up, blue smite them, and they usually die.

PROS:

Good Early Game Dueling
Good Tank
Good Ganks
Decent Clear


CONS:

Tunnel Vision while burrowed
One Crowd Control Ability excluding Blue Smite
If you miss your Knockup, your gank was pretty much useless.

Here are two clears for each side of the map:

Blue Team:Leash Krugs( ), Red Buff, Raptors, Blue Buff( ), Gromp, Wolves, look for ganks

Red Team: Leash Gromp( ), Blue Buff, Wolves, Red Buff( ), Krugs, Raptors, look for ganks.

IF YOU ARE EVER BELOW HALF HEALTH, GET THE SCUTTLER OR BACK.

In Teamfights, your job is to land the knockup on the ADC and then let your team destroy him. You are also the one to tank turrets, Dragon, and Baron. Rek'Sai in general does take a lot of teamwork to pull off, but is beautiful if you can do it right.

Q- Queen's Wrath/Prey Seeker. When Unburrowed, this will make your next three auto attacks do AoE damaged provide more energy. When burrowed, it is a skill shot where when shot at someone it reveals them.
W- Unburrowed, it burrows you into the ground. You have tunnel vision but can see peoples movement. This is SUPER OP as you can see them in bushes, Shaco Q, Rengar Stealth, and more. When you unborrow on a monster or enemy, it knocks them up. This is best used when ganking.
E, when unburrowed it strikes an enemy doing damage. This best used in fights or farming. When burrowed, it makes a tunnel. This is best used when roaming. It is good to put one behind each lanes tower, too.
R, when used, you can teleport to any of your tunnels. Early game, you should use this to get back to farming and be a level ahead of the enemy jungler. Mid to late game, use it to get to teamfights faster.

I hope you enjoyed my Rek'Sai Jungle guide! Any criticism is accepted, and I would like to know what you think! Have a great day!