During her appearance as an enemy character in Book III, the weapon is called Hel Scythe. The weapon's effect is the same (Excluding the "Neutralizes 'effective against flying' bonuses") regardless of its in-game name.
Hel's Reaper/Hel Scythe
Accelerates Special Trigger (Cooldown count-1). Neutralizes "effective against flying" bonuses. If foe does not use magic or staff, calculates damage using foe's Res, and if unit's HP > 1 and foe would reduce unit's HP to 0, unit survives with 1 HP. (Once per combat. Does not stack.)
In Norse Mythology, Hel is the daughter of Loki and is the ruler of the land of the dishonored dead, which is named after her and occasionally called "Helheim."
The Grim Reaper is the personification of the concept of death in many western cultures. Sometimes shortened to Reaper, this personification usually appears as a dark cloaked skeletal figure wielding a scythe.
A Scythe is a common agricultural tool and became associated with the concept of death from the folk saint San Pascualito who was a skeletal figure wielding a scythe.