Something like this? I'm ok with tossing the Will save, but I added the bit in red, since it seems like the goatling might not have attention free to do other things. What do you think?
Discerp (Su): Once per month, a goatling can drain energy from an undead creature or a creature with the Extraplanar subtype. The victim must be physically or magically restrained in some way. The gain a negative level (positive level in the case of undead) each minute that the goatling maintains concentration, and the goatling gains 10 empowerment points. A goatling can discerp a maximum number of levels equal to its HD. If the victim has as many negative levels as HD, it falls unconscious rather than dying. Negative levels are recovered at the rate of 1/day and never become permanent. The save DC is Intelligence(?)-based.
Empowering bit....