Last Stand is an enchantment added by OpenBlocks. It can be applied to any armor piece up to level II.
When a player wearing armor enchanted with Last Stand takes damage that would kill it, the player is instead left with 1 () health at the cost of Experience. The Experience cost is proportional to the prevented damage and decreases with each level of Last Stand, and the final cost never falls below 1 Experience. The effect won't happen if the player doesn't have enough experience to negate all of the damage.
If multiple worn armor pieces are enchanted with Last Stand, their levels are added together when calculating the Experience cost. For example, this means that two armor pieces each with Last Stand I will act the same as a single one with Last Stand II.