Possible Clues:

• An anagram for "rats"
• Anastasia's father, e.g
• Boris Godunov, e.g.
• Onetime Russian ruler
• Ex-leader of Russia
• Bygone Russian despot
• Leader opposed by the Bolsheviks
• Winter Palace monarch
• Leaders of the Spanish Armada rescued emperor
• Ruler opposed by the Bolsheviks