This article's content is accurate to the most recent public game version, 0.4.8d.
"Interrupts the targets' mental energies. Removes all enemies' MP."
Cost: 60 MP
Source: Marianne - Equipping Varek's Core.
Mental Depletion is a spell that removes all MP from all enemies.
Consider using it against Werechameleon (Natalie) just so that she cannot use Grasp of the Pit against Eric(a). Marianne is immune to Instant Death twice over and can just bring Eric(a) back with Bar the Pale Gate, so it is not a great concern either way.