Rengar's abilities have no cost related to them and their use is limited only by cooldowns. Instead, he uses his secondary resource bar to display his unique mechanic: Ferocity.
Rengar builds 1 point of FEROCITY whenever he lands an ability. Upon reaching 5 FEROCITY, Rengar gains the ability to cast an alternate set of abilities, which are empowered versions of his basic abilities. The two ability-sets do not share cooldowns, and his basic abilities' cooldowns will continue to count down while his empowered abilities are available. His empowered abilities have no cooldown and can be used whenever 5 FEROCITY is available, though using an empowered ability consumes all 5 FEROCITY.