Accretive Forcefield

Accretive Forcefield enters the battlefield with X counters on it. For each 1 damage dealt to you, remove 1 counter and prevent that damage.
When you cast a spell, add 1 counter to Accretive Forcefield. It can't have more counters than your devotion to blue.
...if they continued faithful our forces should protect them; but if ungrateful they should be restrained
-Caesar, The Alexandrian Wars