The “ancient grief” was inspired from a fragment of Pindar, quoted in Plato’s Meno, that I read about in the book Ritual Texts for the Afterlife by Fritz Graf and Sarah Iles Johnston:
But for those from whom Persephone accepts retribution
for her ancient grief, in the ninth year she returns their souls
to the upper sunlight; from them arise
proud kings and men who are swift in strength
and greatest in wisdom, and for the rest of time
they are called sacred heroes by mortals.
According to Graf and Johnston, “most scholars are agreed that ‘Persephone’s ancient grief’ refers to the loss of her child, Dionysus, which gave rise, in the long run, to the birth of humanity - and therefore to humanity’s debt to Persephone”. In the Orphic tradition, that is.
In my version Persephone is of course sad for completely different reasons, even if Zagreus/Dionysos is a part of it.