WHat do you reckon was the saddest part of the legendarium?

[Mnemosyne]

Here ends the Silmarillion. If it has passed from the high and the beautiful to darkness and ruin, that was of old the fate of Arda Marred; and if any change shall come and the Marring be amended, Manwe and Varda may know; but they have not revealed it, and it is not declared in the dooms of Mandos.

Simple, bleak, and all-encapsulating.