From Leaguepedia | League of Legends Esports Wiki
Gain 15% (+5% per Bounty Hunter stack) Active Item Cooldown Reduction (Including Trinkets).
Earn a Bounty Hunter stack the first time you get a takedown on each enemy champion.
On Twisted Treeline, each altar counts as a champion takedown.
Cheap Shot damage increased. Ingenious Hunter item cooldown reduction more front-weighted toward initial value.
BASE ACTIVE ITEM COOLDOWN REDUCTION : [10%] ⇒ 15%
ACTIVE ITEM COOLDOWN REDUCTION PER STACK : [6%] ⇒ 5%
This is our first wide balance pass on the Runes paths! We've got a few case-by-case contexts in the tabs below, but are otherwise focused on ensuring no one option in any given row is overpowering its neighbors.
I'LL HUNT YOU : Tooltips for all three Hunter runes now show who you have and haven't killed
ALTAR-NATE HUNTS Each Altar on Twisted Treeline is counted as a potential unique bounty for Bounty Hunter
ALWAYS READY Gain 10% active item CDR plus an additional 6% per Bounty Hunter stack (includes Trinkets)
REKT Earn a Bounty Hunter stack the first time you get a takedown on each enemy champion