As Merlin, successfully hit enemy gods 15 times with a basic attack while Overload is active.
This appears to currently work on minions, as I just unlocked it after barely hitting any of the Overload shots and it still popped. If I could get a second confirmation, it would be appreciated
Overload is Merlin's passive ability. Every time Merlin casts an ability, he gains a stack of Overload. When Merlin next fires a basic attack it will be augmented with lightning, dealing extra damage to the first Enemy.
You simply have to hit an enemy god with the first basic attack after casting an ability, applying the Overload passive. This should be achievable in a single game.
You can build items to increase your cooldown reduction (such as Chronos Pendant or Spear of Desolation) to increase how often you can use an ability.