Hilda is a disguise used by Morgana Pendragon to gain access into Camelot. This disguise is created using an ageing spell Morgana to make her look much older. The ageing spell used by Morgana in order to take the form of an elderly woman is entirely different from the one used by Merlin, as she used no other items in order to take that form, whereas Merlin uses potions whenever he wants to take the form of Dragoon. However, it is shown on many occasions that using this spell exhausts Morgana thoroughly and weakens her power as she painfully transformed back to her original form in order to guard Mithian when she was disguised as Hilda in Camelot. Another problem was it made mounting a horse and walking more difficult for her
Morgana first disguised herself as Hilda in order for her to be able to go to Camelot as Princess Mithian's maid. It is revealed that this spell still has its flaws, as Hilda's pulse and her blood was still that of a young woman, as revealed by Gaius (Another's Sorrow).
Hilda makes another appearance to enter Camelot again so she could talk to Gwen about their plans to kill Arthur, and give her the poison that she was to use (A Lesson in Vengeance).