Rek'Sai
Burrow them in Disgust
Lane harassment and consistent damage output.
Domination
Burst damage and target access
ElectrocuteHitting a champion with 3 separate attacks or abilities in 3s deals bonus
adaptive damage .
Sudden ImpactDamaging basic attacks and abilities deal bonus true damage to enemy champions after using a dash, leap, blink, teleport, or when leaving stealth.
- Eyeball Collection
Collect eyeballs for champion
takedowns . Gain permanent AD or AP,adaptive for each eyeball plus bonus upon collection completion. 
Relentless HunterUnique
takedowns grant permanent out of combatMS .
Precision
Improved attacks and sustained damage
TriumphTakedowns restore 5% of your missing health and grant an additional 20 gold.
Legend: AlacrityTakedowns on enemies grant permanent Attack Speed.
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Electrocute
Rek'Sai is a champion that can quickly and wasily run away with a snowballed early game and the extra burst damage from Electrocute helps you get there with early game kills.
Why Sudden Impact
Sudden Impact massively empowers your early game skirmish and gank damage after tunneling with [E] or casting your [R].
Why Eyeball Collection
Eyeball Collection further empowers your snowball nature letting you run away with your early kills for more damage.
Why Relentless Hunter
Relentless Hunter is the ultimate jungle Hunter rune as the bonus Movement Speed is pivotal in getting to well-timed ganks and fights on time.
WhyTriumph
As Rek'Sai you'll often be diving into the enemy backline for damage and Triumph helps you get out alive after getting the assassination.
WhyLegend: Alacrity
Rek'Sai heavily relies on Attack Speed for jungle clear and [Q] damage so Alacrity is the go-to rune here for a faster clear.
The Terror Beneath
Lane harassment and consistent damage output.
Precision
Improved attacks and sustained damage
Press the AttackHitting an enemy champion 3 consecutive times deals bonus damage and amplifies your damage until you leave combat with champions.
- Triumph
Takedowns restore 5% of your missing health and grant an additional 20 gold. - Legend: Alacrity
Takedowns on enemies grant permanent Attack Speed. 
Coup de GraceDeal more damage to low health enemy champions.
Sorcery
Empowered abilities and resource manipulation
Absolute FocusWhile above 70% health, gain extra
adaptive damage .
WaterwalkingGain
MS and AP or AD,adaptive in the river.
Why Press the Attack
Rek'sai's heavily auto-attack based damage pattern and ability to **auto-reset with [Q]** makes her a great user of Press the Attack. Proccing the exposure damage early in a gank will **empower you and your laner's damage** for a much faster and easier kill.
Why Triumph
Triumph allows you to dive deep into the backline with an [R] and get out safely if you execute them, thanks to the massive heal from Triumph.
Why Legend: Alacrity
More Attack Speed on Rek'Sai mean a **faster Press the Attack proc** and a much **faster/healthier jungle clear**.
Why Coup de Grace
Coup De Grace is a well-rounded reliable damage rune in this slot and **synergizes insanely well with the execution of Rek'Sai's [R]**.
WhyAbsolute Focus
The bonus AD from Absolute Focus is fantastic for empowering your ability to quickly clear the jungle. You should be able to keep the buff active for a large portion of your clear with proper healing from [W] burrow.
WhyWaterwalking
Waterwalking offers every jungle immense neutral objective power, especially when fighting for scuttle crabs.