This article's content is accurate to the most recent public game version, 0.4.8d.
"Deals high Ice damage with a 50% Instant Death chance. No one knows how Marianne learned this curse."
Cost: 12 MP
Source: Marianne - Starting skill.
Freeze the Soul is a spell that deals strong 
Ice damage to a single enemy target with a 50% chance of inflicting 
Instant Death.