Amulet, Annihilation Shield:

When a creature rolls a double 20 attack on you, there will be a 75% chance to negate the second 20 rolled.

Example:
A dragon attacks you, rolling a 20.
The Game Master then rolls again. If another 20 is rolled, you will take x2 damage.
The Game Master roll a D20 and rolls a 20.
You will take x2 damage unless the enchantment of the Annihilation Shield Amulet works.

Value: 54 Gold.