From Nail of Blood:
Inscribed along its length with wicked-looking runes, this 3-inch-long nail is made of black metal and gives off a faint, eerie blue glow. Powered by strange necromantic magic, the nail allows a spellcaster to cast spells within a magic dead zone by drawing on the caster’s life force to power her magic. To be effective, the nail must be placed alongside the flesh in an empty magic item slot (excluding eyes, head, and headband). In a process that takes 1 round, the nail melts painlessly into the wielder’s flesh before taking effect (removing the nail is also painless and takes 1 round). Once the nail is in place, its wielder can cast within a magic dead zone up to one spell of each spell level she knows or has prepared and could normally cast. If she wishes to cast additional spells in the magic dead zone, she can do so up to her normal limit, but each additional spell she thus casts causes her to become fatigued and requires a successful concentration check (DC = 15 + the spell’s level) or the spell fizzles. If the nail of blood would cause the wielder to be fatigued when she already has the fatigued condition, she becomes exhausted. She can’t use the nail of blood to cast additional spells while she is exhausted, no matter what effect caused her exhausted condition.
What is a magic dead zone?
From what I've seen while searching, it's apparently different from an antimagic field or a dead magic plane. So, what is it, and what/where are the rules for it?