Executioner
CondDmg
Bonus damage while the enemy is below 30% HP.
Rolls between 8% and 45%, scaling with the gear score of the item carrying it.
Forge mods that grant it
Spending one of these puts the perk on your gear instead of waiting for a roll.
Rolls on
SwordWandStaff
A drop or a craft of these types can roll this perk into one of its slots.