You gain +1 AC.
In addition, once per battle when a nearby ally is hit by an attack, you can choose to lose hit points equal to half of that damage, and have your ally take only half of the damage instead. The damage you lose can come from temporary hit points, but isn't affected by damage resistance and other tricks to avoid the damage.
Increase your total number of recoveries by 1.
Once per day, you can use Bastion twice in the same battle.
When you use Bastion now, your ally takes no damage. You still lose hit points equal to half the damage.