League of Legends Reforged Rune: Aftershock
Aftershock
After immobilizing an enemy champion, increase your Armor and Magic Resist by 35 + 80% of your Bonus Resists for 2.5s. Then explode, dealing magic damage to nearby enemies.
Damage: 25-120 (+8% of your bonus health)
Cooldown: 20s
Resistance bonus from Aftershock capped at: 80-150 (based on level)
Rune Path
Resolve
"Immobilize" in this context means "prevents the player from controlling their champion's movement," which is why Polymorph doesn't proc Aftershock, since you can still control where you move.