Executioner's Arc
Proc
Kills grant a guaranteed crit on your next hit.
Rolls between 10% and 40%, 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
Greatsword
A drop or a craft of these types can roll this perk into one of its slots.