Ageing Spell

An Aging Spell allows the sorcerer who casts it to grow older. It's not clear if the warlock or witch can control the spell and decide how much to get older. When Merlin used this spell, he turned into an 80-year-old man, but he had declared it was his intention to get this old. Aging Spell are notoriously difficult and unpredictable and sometimes they don't hold for long.

When Gwen was arrested for witchcraft, Merlin suggested that if another culprit were caught planting an identical poultice to the one found in Arthur's chambers, then Gwen would have been freed. In his chambers, he prepared an unknown powder that threw into a cauldron while he chanted: "Mihte dagan, bebiedaþ me. Adeadaþ þisne gast min freondum ond min feondum." The powder exploded and as white smoke rose to his face, Merlin transformed himself into an old man.

Later on Merlin tried to break the spell with the incantation "Edniwe min geoguð!". Anyway he failed because the spell was too powerful. Only thanks to Gaius' antidote, Merlin was able to turn back into himself (Queen of Hearts).

It is possible that Morgause used an Aging Spell as well to take on the form of a poor old beggar woman and to meet Morgana in the city market. Anyway her spell wasn't as powerful as Merlin's, for her true form could be revealed in mirrors (The Eye of the Phoenix).