Mithral Skeleton (3.5e Feat)
From D&D Wiki
Mithral Skeleton [Necromantic]
Undead you create have their skeleton structure reinforced by mithral
Benefit: When you create an undead creature you may choose to add 100 gp worth of Mithral and an emerald worth at least 75 gp to the material components of the spell. If you do, the undead creature gains 6 hit points per Hit Die. A skeleton created with this feat also gains +6 natural armor, +6 dexterity, +10 ft/round speed and gains +2 to hit, and deals +2 damage with its claw attacks. An undead created in this way counts as three Hit Die higher for the purposes of creating it and controlling it. You can only raise an undead creature's effective Hit Dice to one half your caster level through Necromantic feats.
Special: This feat may only be applied to undead that have an intact skeleton (skeletons, zombies, ghouls, but not ghosts, for example). You may not apply this feat to an undead in conjunction with any other feat that reinforces its skeleton.