Arcane Genetic Alchemy

WHEN: Your opponent's Shooting phase or the
Fight phase, just after an enemy unit has selected
its targets.
TARGET: One Adeptus Custodes Infantry unit
from your army (excluding Anathema Psykana
units) that was selected as the target of one or
more of that enemy unit's attacks.
EFFECT: Until the end of the phase, each time
an attack is allocated to a model in your unit,
subtract 1 from the Damage characteristic of
that attack.