Lesser Invocation; 4th; Eldritch Essence
This eldritch essence invocation allows you to change your eldritch blast into a hellrime blast. A hellrime blast deals cold damage. Any creature struck by the attack must make a Fortitude save or take a –4 penalty to Dexterity for 10 minutes. The Dexterity penalties from multiple hellrime blasts do not stack.
Source: Complete Arcane