Necromancer’s Death Shroud 1 (underwater!) with the trait “Unyielding Blast” causes vulnerability on your necromancer (in addition to the target). It’s only supposed to affect your target, not your necromancer.
This causes underwater Life Blast underwater to actually be MORE powerful because the underwater version transfers that conditions to your foe, causing you to inflict double the vulnerability. You’ll get to 25 stacks of vulnerability on an enemy in just a few attacks.