Dawn of Darkness was a cancelled sourcebook for Ars Magica. It was cancelled after Ars Magica was sold to Wizards of the Coast in 1994 and how little Wizards produced under 4th edition before the license was eventually sold to Atlas Games.
Summary[]
From the White Wolf 1993-94 Catalog:
"At long last the truth behind mysterious House Diedne is revealed. What terrible secrets lay at the House’s destruction centuries ago? What terrible evil have the House’s survivors been brewing? What horrible revenge waits to be sprung upon an unsuspecting Order of Hermes? This long-awaited supplement answers all these questions and may change the very direction of your Ars Magica Saga." - pg. 26