King Polydectes wanted to get rid of Perseus so he sent him out to retrieve the head of Medusa.
The Nymphs of the North
The goddess Athena and the god Hermes came to Perseus while he was on his journey to Medusa , Athena lend him a shield to protect him from Medusa stare and Hermes gave him a sword to cut off her head.
The Lady's only had 1 eye to share but they were the only ones who knew where the land of the nymphs where so Perseus took the eye while they were passing it and forced them to tell him where the land of the nymphs were. they were so frightened, they told him instantly and Perseus gave the eye back and ran off.
Finally, Perseus came upon the Nymphs of the North. The Nymphs greeted them and gave him the last three things he needed: a pair of winged sandals, a cap of darkness, and a magic sack which he put the head of Medusa in.
Perseus flew on his winged sandals to the island where Medusa was at. He was very careful to only look at Medusa's reflection from the shield he had. When he found Medusa, with just one stroke, he chopped her head off. He put her head in the magic sack and flew away.
Once Perseus returned back to Seriphus, he discovered that King Polydectes only wanted to get rid of him. With rage he went to the throne room to find him. When he did he grabbed the head out of the sack and shouted "Here is the head you wanted." and the King was instantly turned into stone.