A diamond of protection is a protective shield that can defend against enemies and especially the Dead, as their body will burn up when they come into contact with it. However, it can only take so much damage: Orannis, in his second form, annihilated three nested diamonds of protection in barely a minute.
The diamond of protection is cast by drawing four specific Charter marks on the ground: the North Mark, the East Mark, the West Mark and the South Mark. When the diamond is complete, it appears as four golden, burning points linked by golden lines. These are hot enough to melt snow.
The Abhorsen is more fortunate than Free Magic necromancers, as leaving one's body to spiritually enter Death leaves one extremely vulnerable. Without a diamond of protection -- which is Charter Magic, anathema to Free Magic -- necromancers have to rely on servants or keepers to guard them.
Kerrigor lured Terciel to the reservoir at the Royal Palace in Belisaere, and Terciel entered Death, leaving his body defenseless. However, the diamond of protection he had cast beforehand remained so strong for so long (possibly weeks) that Kerrigor was forced to trap him in Death instead, luring Terciel's daughter Sabriel to the reservoir in turn.
It is possible that drawing using a Charter-spelled item, like Abhorsen's sword, increases the strength or ease of this spell.