D&Dtools1

You cause the skeleton of the target corporeal undead to become thicker and as strong as stone.
This gives the target a +3 natural armor bonus to AC.
It has no effect if the creature has no skeleton (such as an undead plant or octopus) but works normally on creatures that have exoskeletons (such as insects).
Another version of this spell is rumored to exist that works on living creatures, but causes wracking pains because of the radical alteration of its skeleton.
The other version has fallen out of use because few creatures willingly subject themselves to it (except perhaps worshipers of Loviatar).
Arcane Focus: A miniature skull carved of granite.