Undead you raise or create are tougher than normal.
Benefit: Each undead you raise or create with any necromancy spell gains a +4 enhancement bonus to Strength and +2 hit points per Hit Die.
Is a prerequisite for: Bolster Resistance - Undead gain +4 turn resistance
Deadly Chill - Undead deal +1d6 cold damage on melee attacks
Destruction Retribution - Undead deal negative energy damage when killed
Hardened Flesh - Undead gain +2 natural armor
Nimble Bones - Undead gain +4 to initiative and +10 ft./round.
Source: Libris Mortis