Shadow Hag is the boss of the third dungeon in Oracle of Ages, Moonlit Grotto. Shadow Hag is basically a witch-like figure that can sink into the ground like a shadow. Shadow Hag will start off the battle by splitting into four circular shadows which will move quickly around the room. These can be difficult to avoid, but you can run away from them or jump over them using the Roc's Feather. After the shadows join together, Shadow Hag will reform and run at you from behind. The important thing is to not look at her (if you do, she'll form back into the four shadows), and instead use the Seed Shooter to launch seeds at the wall in front of you, where they will bounce off and hit her. Occasionally she will also send out butterflies which you can kill to get more seeds. After about six hits, Shadow Hag will be defeated.
The Shadow Hag does resemble Joelle, Who was one of the four Poe Sisters in both Ocarina of Time and Majora's Mask.