Merlin first cast such a spell on the Isle of the Blessed, where is powers of creature of the Old Religion were heightened. He was able to master the power over life and death and did this to strike the priestess Nimueh in order to restore the balance of the world through her death. By raising his hand, he gathered the clouds above his enemy and summoned the gods to strike her with incredibly powerful magic (Le Morte d'Arthur).
During the Battle of Camlann, Morgana, last remaining High Priestess of the Triple Goddess, performed a very strong stunning spell with a golden flash of her eyes, which created a vivid white light and hit all the Camelot knights that had surrounded her champion Mordred, killing them instantly.
Emrys, in his aged form, used his magical staff to create destructive lightning bolts and send them, from a cliff overlooking the Plain of Camlann, on Morgana's army, causing an incredible number of victims. He was even able to strike Morgana, although she was only knocked unconscious for a while (The Diamond of the Day: Part Two).