Darillium

Darillium was the location of the Singing Towers. The Doctor took River Song to see these towers the last time she saw him before she went to The Library. She states says, to the Doctor that "The towers sang, and you cried, but you wouldn't tell me why. I assume you knew it was time? My time. Time to come to the library." which ties into Last Night'' as the second version of the Eleventh Doctor tells the first version of the Eleventh Doctor that he is taking River there and they discuss how it is her last meeting with them (DW: Forest of the Dead, DW: Last Night)