The Augmentation spell was a powerful spell that altered reality, including the perceptions and memories of living beings. The spell turned the caster into a paragon, the best of everything, and only him/her would be aware of the changes. However in order to balance the new force of good, the spell also created an opposing force of evil, everyone's nightmare. The spell could be broken by killing the force of evil.
The spell required the caster to scar him or herself with a mark, which was also bore by the Monster. The death of either the caster or the balancing creature would undo the spell, undoing the alterations made to reality and restoring the memories of those affected by the spell.
Jonathan Levinson cast the spell on the year 2000, having learned it from someone at therapy sessions he had attended since his suicide attempt the previous year. Adam, due to his unique self-awareness, was aware of the alteration of reality.
One of the affects of the spell was it made Jonathan stronger than Buffy but when the monster was beaten on, he lost this strength. The spell was broken when Jonathan killed the monster to save Buffy.