Water Elementals are beings of pure water. They are immune to cold, but vulnerable to fire.
Summon - Unit was summoned and belongs to another world.
Illusion Immunity - Unit is immune to illusions. It can see invisible units and illusory attacks do not ignore its armor.
Cold Immunity - Unit is immune to Cold damage.
Elemental - Unit is immune to Mind-affecting effects, Critical hits, poison and sneak attacks.
Evasion - A successful reflex saving throw that would normally prevent half damage prevents all damage instead.
Fire Vulnerability - Unit suffers 100% additional damage from all Fire attacks.