Thunderlord's Decree's cooldown is general (not per-target)
Trivia
Deathfire Touch will apply more damage on one single-target spell (i.e. over 4 seconds) than Thunderlord's Decree if a champion has more than 573
bonus AD or more than 1146 AP.
Patch History
V7.22
Removed due to Season 2018.
Replaced by
Electrocute.
V6.11V6.2
Cooldown changed to 25 − 15 (based on level) seconds from 20 at all levels.
Stack
duration reduced to 3 seconds from 5 seconds.
V5.24
AD ratio increased to 30% bonus AD from 20% bonus AD.
V5.23
Cooldown reduced to 20 seconds from 30.
November 17th Hotfix
Bug Fix: Fixed a bug where the per-target cooldown was unique across multiple champions with the mastery.
V5.22
Added
Tier 6 Cunning Keystone mastery with 1 rank.
Hitting an enemy champion with an ability
(ability damage) or basic attack (on-hit) marks them with Thunderlord's Decree for 5 seconds. At 3 stacks, the target takes 10 − 180 (based on level)(+ 20% bonus AD)(+ 10% AP)
magic damage. Thunderlord's Decree can only be triggered once every 30 seconds.