Channeling
RampHit
Damage ramps while you keep hitting one enemy.
Rolls between 1% and 5%, 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
WandStaff
A drop or a craft of these types can roll this perk into one of its slots.