Act II: Gorgon Reference

Gorgon were the three sister in mythology who are beautiful and tall. When Athene found out that her husband, Poseidon, was cheating her on her with on Medusa, one of Gorgon sister. Athene turned Medusa's hair into snakes and now everything that looks at her will turn into stone. 

When Macduff said, “Approach the chamber and destroy your sight/ With a new Gorgon…” he meant that King Duncan looked like Medusa, ugly. Macduff saw Duncan after he was killed and stabbed to death and this is a reference to what Medusa looked like after Athene made her look ugly and turned her into a monster.