This article's content is accurate to the most recent public game version, 0.4.8d.
"[5 Turns] Casts a debilitating mass-curse. Lowers all targets' ATK and MAT."
Cost: 6 MP
Source: Equipping the Death Mist Core.
Withering Curse is a spell that applies ATK and MAT Down for five turns to all enemies.
As a debuff spell covering both ATK and MAT for such a long time it is useful against any high damaging bosses to increase the party's survivability. Stacking it with defensive buffs on the party would lower incoming damage even further.
The following enemies can cast Withering Curse: