Harpy ooze is the vaginal secretions of Harpies and is also the slipperiest substance in existence. As such, it is the ultimate marital aid, especially suited for well-hung humans who marry elf women with tiny vaginas. The exploits of one such human in his quest for harpy ooze are depicted in the documentary "The Elven Bride" .
Regrettably, Harpy ooze is difficult to acquire due to the fact that Harpies are quite rare.
Fortunately, in modern times, the invention of anal lube has helped many people who would otherwise have had to seek out Harpy ooze.
As Oscar Wilde once said, "Harpy ooze helps me penetrate even the tightest elven vaginas!"