From the back of the book:
Birth. Growth. Death. Rebirth. The cycle ever continues. Without but one cog in the wheel the mill of life shudders to a halt. Without birth there is no growth. Without death there is no rebirth. So it is for your Covenant. The four seasons pass before you, but without the death of cruel Winter, your Covenant cannot be reborn into gentle Spring. Steel your grip on life and death means your tale's end. Accept your fate and your Saga is told anew.
Twelfth Night is an Ars Magica 3rd edition Saga supplement, the fourth an final chapter in the Four Seasons Tetralogy. In this sprawling epic, characters and their decrepit Winter Covenant must endure the wild machinations of mad wizards and the onslaught of righteous Crusaders, only to face death itself. For without its demise the Covenant cannot be reborn. Without its death your Covenant cannot return to Spring.