Catena Aurea on Luke 1:79: Gregory the Great

But the shadow of death is taken to mean the forgetfulness of the mind. For as death causes that which it kills to be no longer in life, so whatever oblivion touches ceases to be in the memory. Hence the Jewish people who were forgetful of God are said to sit in the shadow of death. The shadow of death is taken also for the death of the flesh, because as that is the true death, by which the soul is separated from God, so that is the shadow of death by which the flesh is separated from the soul. Hence in the words of the martyrs it is said, the shadow of death has come over us. (Ps. 44:19.) By the shadow of death also is represented the following of the devil, who is called Death (Rev. 6:8.) in the Revelations, because as a shadow is formed according to the quality of the body, so the actions of the wicked are expressed according to the manner of their following him.