Glimpse of the Void is a buff received upon slaying the Rift Herald, located on the Summoner's Rift. The buff can be identified by swirling violet runes around the champion. Runic Affinity increases the duration of Glimpse of the Void by 3 minutes.
While there are no allies nearby, you take 5% damage reduction from enemy champions and generate X / Y Corruption stacks per second. At 100 stacks, your next basic attack consumes all stacks to deal 15 / 270 bonus magic damage. Works versus structures, damage halved for ranged champions.
Lasts for 20 minutes and is retained even if slain.