Electrocute

Effect

Passive: Damaging basic attacks, abilities, item effects, and summoner spells, as well as the application of crowd control and damage over time effects, apply stacks against enemy champions, up to one per cast instance per champion. Applying 3 stacks to a target within a 3 second period causes them to be struck by lightning after a 0.25-second delay, dealing them 70 to 260 (+ 10% bonus AD) (+ 5% AP) of either physical or magic damage.

Variable Damage: This effect deals either physical or magic damage depending on the damage contribution from your attack damage and ability power to the effect's damage formula.
Greater bonus damage from the AD ratio → Physical damage
Greater bonus damage from the AP ratio → Magic damageIf the damage contribution of AD and AP are zero or otherwise equal, the damage type defaults to magic damage.