This is likely a reference to the 1940 Katyn massacre: After the Soviet invasion of the Eastern half of Poland (while the Nazis took the Western half) over 20,000 Polish military and police officers, border guards, and intelligentsia were rounded up, executed and buried in mass graves by the NKVD.
It was actually discovered by the Nazi after they broke their Non-Aggression pact with the Soviets, and used it to discredit them after the Allied Powers reluctantly accepted them. The Soviets blamed the Nazis, and continued to deny being responsible until 1990, by which point they and later Russia go "Everyone involved is dead, so no further investigation or cases."
I've already noticed the White Elves uniforms look like the uniforms of the Russia Empire/Soviet Union, so I wouldn't be surprised if that's the direction they take.