"The Nightingales are creatures that have existed since time began immemorial, feeding on the weak, ill, and dying in wars and natural disasters. Patrolling WWI era trenches, you discover that you are more than just at war…you’re being hunted."
Only so many wars you can do, especially ones that aren't touchy subjects. WWI is one of the least touchy wars, it seems. Vietnam is obviously EXTREMELY touchy, as is the Civil War.