From the publisher's website
Live the Legend. The covenant of Fanum has broken the Code of Hermes by interfering with the mundane population of Germany. Their interference has resulted in the near-destruction of another covenant, and now the members of Fanum have been summoned before the Rhine tribunal to face Hermetic justice.
You are that tribunal. The Fallen Fane is a live-action Ars Magica scenario for 12 to 24 players. It's equally suitable for Ars Magica veterans and those who have never played the game before, and can be run in an afternoon.