Necklace of Bloody Barrier

This necklace was taken of the body of Arianne.
A blood red necklace in the shape of scaled blood drops, the blood drops seems to made from actual stilled and solidified blood and feel unnaturally durable. It is said than an enslaved blood elemental was forcefully bound to the necklace and then the jewelery was enchanted by abjuration magic for 7 days.
The necklace bears the signature mark of “The Ghoul”.
When hit by an attack you can see, as a free action, you can pay the price of blood to raise you AC to be able to block the triggering attack.

• The first time you use the ability, the price is the difference in AC as blood (pure damage).
• Every use afterwards increases the cost.
• The second use is 2x the difference, the third use is 3x the difference and so on.
• The counter resets 1d6+1 charges dawn.

Limitations:

• You must be able to see the attack coming.
• The damage taken from the item cannot be negated and ignores temporary HP
• When the attack is from a spell, you add the spell level to the difference sum.
• The blood can only take you to 25AC max, when you take it higher the necklace will block the attack but deactivate until dawn.
• When used in such a manner, roll a d20, on a 1 the necklace shatters.
• A critical hit cannot be blocked.