A Regression Spell was a spell that allowed a mancers body to revert to its youth for a very brief time. Unlike renewal, a regression spell did not copy and renew the bodies cells; a regression spell was temporary and the peek state of youth possible for the spell lasted for perhaps a month, while after a mancer had used renewal the changes were permanent and the ageing process remained as it would if the body truely was that youthful.
After a month at the peek state of youth, the body slowly and painfully returned to its true state. The regression spell was also unable to allow a person to become fertile once more. A regression spell was a powerful and potentially fatal use of the Secret Art that was of the same branch of magic as renewal.
Tulitine used a regression spell in 222 ATM, however, the reversion of the spell was interrupted by the alien forces of a caduceus which caused the spell to revert abnormally; and indeed, after the caduceus was destroyed Tulitine's body remained as youthful as it had when the spell was interrupted.