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. While the swing is readied, opponents whose damage is at or above 70% hit with the tip of a smash attack will be surrounded by a darkness effect, at 69% or less if hit with any part of the death scythe the blade of the scythe will have a red effect. Hitting such a heavily damaged opponent with the tip of Death's Scythe during a smash attack, or even a tilt attack if their damage is at or above 120%, will instantly KO them. Characters KO'd in this manner do not take knockback, and instead simply vanish in a colorful explosion.
Equipping the Antasma, Mimikyu, Nash or Hector (Castlevania) Support Spirits will give the player a Death's Scythe at a start of a match.