Lightning was a spell acquired and used by Kain in Blood Omen: Legacy of Kain. It was considered a secret spell, as it was found behind a moon gate by Kain, right after visiting the Dark Eden and is one of the last spells of the game. It is the most powerful and costly spell in Kain's repertory, as it enables him to call upon thunderbolts from the skies, to crush all enemies onscreen with a massive amount of energy along a period of time, but due to being related to the background, this spell can only be cast outside in open areas. Bosses can't be injured with this spell.
The passage that led to the Lightning spell was populated by many Werewolves who had apparently entered through the moon gate on a full moon and become stuck.
The Futhark runes outside the Lightning passage hints at the moon cycles and gates, an important element of the game for unlocking the 100 Secrets. They translate as: "He will come when the moon is full and learn to call death from the sky"
Though only explicitly seen in Blood Omen, later games in the series featured several more focused 'lightning' attacks; with Kain apparently learning to focus this spell and fire it from his hand, using it against Raziel with a deadly effect (as it can instantly dissolve the latter's physical body or take half of his health if he has acquired all of the Health Upgrades) in the Chronoplast at the end of Soul Reaver and is also used at the beginning cinematic of Soul Reaver 2 . In the same fashion, the Hylden Lord uses a similar lightning related spell against Kain's vampire army during the introduction cinematic of Blood Omen 2.