Sources: Dragon #312
Dragon Compendium Vol. 1
You can add such a powerful dose of necromantic energy to your mind-affecting spells that they function against undead creatures but are useless against all others.
Benefit: A mind-affecting spell modified by this feat works normally against intelligent undead creatures. Mindless undead (those without Intelligence scores) are still immune to its effect, and the altered spell has no effect against living creatures or constructs. A song of the dead spell takes up a spell slot one level higher than the spell's actual level. Note that use of this feat does not make mind-affecting spells affect undead if the spell's description specifies that the target must be living or a particular creature type (other than undead).
Any spells prepared with Song of the Dead become necromancy spells.