Least Invocation; 2nd; Eldritch Essence
This eldritch essence invocation allows you to change your eldritch blast into a sickening blast. Any living creature struck by a sickening blast must make a Fortitude save or become sickened for 1 minute. A sickened creature struck by a second sickening blast is not affected by the sickening aspect of the blast but still takes damage normally.
Source: Complete Arcane