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, similar to a poison. 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.