The user can kill the target with a single strike. The attack carries special attributes, such as lethal poison or death inducement, that would result in an instant death the moment it connects, disregarding on the location of contact or the target's remaining vitality.
Other, less sophisticated methods are either the attack strikes the target's vital point, or it just carries devastating firepower and brute force to outright kill the opponent.
The instant kill is capable of bypassing regeneration of anylevel, as it deals death before the opponent has a chance to rejuvenate.
Peter Pettigrew (Harry Potter) uses the Killing Curse, reputably unstoppable and irreversible, to instantly kill Cedric Diggory.
Akame's (Akame Ga Kill!) One Cut Killer: Murasame kills any it cuts with a cursed poison, even the user if not careful while cleaning the blade.
White Bomber (Bomberman 64: The Second Attack) detonating a Gravity Bomb, which will kill any and all it sucks in instantly, including White Bomber if he's not careful.
The moment Itachi's (Naruto) Sword of Totsuka pierces a target, it is sealed into a "world of drunken dreams" inside the sake jar, for all eternity.
Kaguya Ōtsutsuki (Naruto) firing her All-Killing Ash Bones that will cause her victims to disintegrate into ashes the moment it pierces the body.
Magellan's (One Piece) Kinjite poison spreads like disease through organic material and rocks, and kills instantly the moment it infects the target.
Link's (The Legend of Zelda: The Wind Waker) Light Arrows will instantly kill any evil forces it pierces, bypassing any defenses and vanquishing it in pure light.
Gae Bolg (TYPE-MOON) divides into countless thorns that destroy target's innards when it pierces the heart.
Certain weapons the Inklings (Splatoon) use deal enough damage in one hit to "splat" their enemies instantly.
Agent 3 (Splatoon)
Certain weapons the Octarians (Splatoon), such as DJ Octavio, use deal enough damage in one hit to "splat" Agent 3 instantly.
The Vorpal Sword from Jabberwocky by Lewis Carrol can kill the Jabberwocky with one hit. In other lore, it can kill anything with a single cut.
Alan’s (Marchen Awakens Romance) superhuman strength lets him even destroy enormous monsters with a single punch.
Machina's (Dies Irae) Briah, Midgardr Völsunga Saga, allows him, with merely a single blow, to erase the existence of anything with a history, including living beings, inanimate objects and even concepts.
Rose's Sword (Steven Universe) was made to be able to destroy a gem's physical form with a single swipe.