Death's Scythe is, as the name implies, the weapon that accompanies Death in the Castlevania series. It is said to reap souls from those it slashes, and can sometimes even be used by the playable characters; it is usually considered one of the most powerful weapons of the series.
In real life and other forms of media (including books, cartoons, and other video games), the personification of Death (or, as it is more commonly known, the Grim Reaper) is usually depicted with a scythe as its primary weapon and tool in order to gather souls from those who have passed on.
Death's Scythe is a battering item which the user swings in an arc ahead of them. If the opponent's damage is high enough, it will instantly KO them. While the swing is readied, opponents with sufficient damage will be surrounded by a darkness effect. If the opponent's damage is not high enough, and they are hit with any part of the scythe, the blade of the scythe will have a red effect. Hitting a heavily damaged opponent with the tip of Death's Scythe during a smash or tilt attack will instantly KO them. Characters KO'd in this manner do not take knockback, and instead simply vanish in a colorful explosion.
The minimum damage required for the Death's Scythe to instantly KO is 70% for a fully charged smash attack, 80% for an uncharged smash attack, and 120% for a tilt attack. Other attacks with the scythe (e.g. jab, dash attack) will never instantly KO the opponent.
Equipping the Antasma, Mimikyu, Nash or Hector (Castlevania) Support Spirits will give the player a Death's Scythe at a start of a match.