Rengar
Carve Off a Souvenir
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.
Grisly MementosCollect 1 memento on champion
takedowns , up to 18 total.
Gain 6 Trinket Haste for each collected.
Ultimate HunterUnique
takedowns grant permanent cooldown reduction on your Ultimate.
Sorcery
Empowered abilities and resource manipulation
CelerityAll
Move Speed bonuses are 7% more effective on you and gain1% Move Speed .
WaterwalkingGain
MS and AP or AD,adaptive in the river.
Adaptive Force
+9
Adaptive Force Adaptive Force
+9
Adaptive Force 
Health Scaling
+10-180 Health (based on level)
Why Electrocute
Electrocute is a fantastic assassination rune on Rengar thanks to his ability to almost **instantly proc** it with a Leap->[Q]->[W]. The extra burst allows you to secure your kills in a single rotation of spell.
Why Sudden Impact
Sudden Impact is a no-brainer on Rengar. He is a **natural abuser of dashes** thanks to his passive. With good positioning around bushes you should be able to keep the buff active at all times granting you permanent free Lethality.
Why Grisly Mementos
Grisly Mementos is a safe a reliable rune on every Jungle thanks to the nature of their vision control and ability to deep ward.
Why Ultimate Hunter
Ultimate Hunter is a great replacement for Ultimate Hat on Rengar, allowing you snowball successful [R] gank leads into more kills in the mid game.
WhyCelerity
Celerity's Movement Speed and bonus AD on Rengar is great for empowering your ability to content early scuttle crabs and scrap for fights in the early game.
WhyWaterwalking
Waterwalking further empowers your ability to contest scuttle crabs and scrap for fights in the early game as well as synergizes well with the extra movement speed from your [R].