Bats of Mercille
From Project: Redcap
The Bats of Mercille was an early scenario for Ars Magica. It was published by Lion Rampant in two small print runs, for ArM1 and ArM2. This supplement is probably the rarest collector's item in Ars Magica's history.
Subject and Contents
All we really know about The Bats of Mercille comes from its page at rpg.net. Rather than plagiarize that article, it's better to just tell readers to go there.