D&Dtools1

You can modify your breath weapon so that it is laced with negative energy.
In addition to the normal energy damage your breath weapon deals, creatures that fail their saving throws against the breath weapon gain 2d4 negative levels.
The modification applies only to the breath produced as part of the casting.
If the subject gains at least as many negative levels as it has Hit Dice, it dies.
Each negative level gives a creature the following penalties: -1 penalty on attack rolls, saving throws, skill checks, ability checks, and effective level (for determining the power, duration, DC, and other details of spells or special abilities).
Additionally, a spellcaster loses one spell or spell slot from her highest available level.
Negative levels stack.
Assuming the subjects survive, they regain lost levels after a number of hours equal to your caster level.
Ordinarily, negative levels have a chance of permanently draining the subject's level, but the negative levels from enervating breath don't last long enough to do so.
If an undead creature is caught within the breath weapon, it gains 2d4×5 temporary hit points before taking damage from the breath weapon.
If not immediately lost, these temporary hit points last for 1 hour.