How to make undead enter negative hitpoints

[Demonic_Spoon]

I seem to remember some sort of feat or such that could allow undead to enter negative hitpoints and still be "alive" or animate as it were. Could anyone tell what this feat is if it does exist? If not are there any other methods to make them enter negative hitpoints and still be animate?

EDIT: Nevermind, found it, necrotic reserve in LM.