Arcane Genetic Alchemy

Just after an anemy unit has selected its target.
{x} One Adeptus Custodes INFANTRY unit
from your army (excl. Anathema Psykana)
that was selected as the target of one or more
of that enemy unit's attacks.
{t} Until the end of the phase, each time an
attack is allocated to a model in your unit,
substract 1 from the Damage characteristic
of that attack.