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 says, in the Library, to the Doctor that "The last time I saw you- the real you, the future you, I mean- you turned up on my doorstep with a new haircut and a suit. You took me to Darillium to see the singing towers. What a night that was. The towers sang and you cried. You wouldn't tell me why, but I suppose you knew it was time. My time. Time to come to the Library. You even gave me your screwdriver. That should have been a clue." This 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)