Takedowns on enemies grant permanent Attack Speed.
Offense
+9% Attack Speed
Flex
+5 AD or 9 AP
Defense
+6 Armor
WhyElectrocute
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.
WhySudden Impact
Sudden Impact massively empowers your early game skirmish and gank damage after tunneling with [E] or casting your [R].
WhyEyeball Collection
Eyeball Collection further empowers your snowball nature letting you run away with your early kills for more damage.
WhyRelentless 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.