Milton refers to Moses as Amram's son in order to convey the importance of the relationship between Moses and Satan.  Amram was considered as the friend of Jehovah.  It is evident that Moses is a very religious figure because Amram is considered very close to God and Milton uses the relationship between Amram and Moses to refer to Moses.  Satan is compared to Moses in order to covey the control that he had over his minions.  Satan calls his army forward and as they swarm Milton compares the image of Moses' plague of Locusts swarming over Egypt, and the comparison to Moses implies a powerful control over the swarm of angels.  Milton uses this reference to Moses to convey the power Satan held in Hell over his angels.  Moses is seen as such a pious figure that idea of Satan compared to him is contradictory.  Milton uses this contradiction to further convey Satan's impious character.

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