Foe Requiem is a series of songs in Final Fantasy XI. They affect a single enemy. There are 7 versions of the song, I through VII, and all of them have the effect of a very light damage over time. Foe Requiem is highly difficult to resist, and can only be removed by Erase. The only downside is that it is not very powerful.
The duration can be increased and a requiem's resistance can be decreased if a character has equipped a Flute, Siren's Flute, or Shofar when singing the song.