Overcharge
EveryNth
Every Nth cast deals greatly increased damage.
Rolls between 8 and 4, 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.