Transmutation
Level: Cleric 4, Sorcerer 4, Wizard 4
Components: V, S, F
Casting Time: 1 standard action
Range: Touch
Target: Corporeal undead creature touched
Duration: 10 minutes/level
Saving Throw: Will negates (harmless)
Spell Resistance: Yes (harmless)
A brief flash engulfs your undead ally, and through its flesh you can see its skeleton. The skeleton glows a dusky red for a moment.
This spell functions like stone bones, except that the subject creature’s skeleton changes to iron. The creature gains a +6 natural armour bonus to AC.
Arcane Focus: A miniature skull made out of iron or steel.
Note: Does this need a Divine Focus?
Source: Spell Compendium (Page 125)