"Why would you want to borrow anything from The Void?" you might think. "Does it have a library now or something?" That is because you're not paying attention. I said BURROWER not borrower.
Rek'sai is a monstrous former Void inhabitant come to wiggle across and underneath the Shuriman sands devouring nomads and generally being a terrible neighbour. "When we started working on Rek'Sai it was with two concepts in mind: Burrower and Land Shark," is how Riot put it. I'm currently looking at her and trying to decide what her pose reminds me of and I've realised that although she looks closer to a scorpion, her stance is closer to that of an earthbound dragon – you know, when they're all furled wings and elbows.
Anyway, what's her deal? It's mostly about burrowing. While underground she gets Tremor Sense which shows all enemy movement in an area around her (so she's kind of a mobile ward capable of picking out or just avoiding foes as they run about), loses a bit of vision range and gains movement speed. When unburrowing she knocks up and damages surrounding enemies. Some of her other abilities also alter depending on whether she's over or underground at the time. Laying tunnels is key as her ult lets her charge towards them from anywhere on the map.
Riot are billing her as a jungler, fit to frolic in their newly renovated jungle:
"Clever use of her tunnels gives Rek’Sai unparalleled mobility through her jungle and into the enemy’s domain. Once there, she works best counter-jungling her opponent into obscurity before ganking enemy laners from the unconventional angles her tunnels provide."