Memory loss. Characters cannot remember anything that happened before the disorder took hold. Memory loss is total - they forget their name, life history, and any spells. Skills are not forgotten, but the character will not realise that they are available unless reminded by another character.
To overcome amnesia, characters are allowed to make an Int test once per month - at the GM's discretion, a further test might be made after any traumatic experience, such as a blow on the head. When the test is passed, the character can recall everything except the events which led to the affliction. The character may, if the player wishes, make further monthly Int tests to recall these events, but will gain one insanity point for each failed attempt.
During a period of amnesia, a character is allowed to spend experience points to re-learn forgotten spells. When the memory returns, however, these experience points will have been wasted.