Historical clue formulations

• Darken — 2015-09-20
• Obscure — 2013-12-21
• Darken — 2013-03-31
• Darken — 2012-10-26
• Obscure — 2008-02-10
• Shroud — 2007-07-15
• Obscure — 2004-11-14
• Obscure — 1999-02-04
• Obscure — 1999-01-16
• Obscure — 1998-09-26
• Obfuscate — 1998-05-17
• Obscure — 1995-04-06