This concept is based upon the Ziz which was supposedly the greatest of all creatures in the sky, and was associated with the leviathan and the behemoth, who corresponded with the sea and land respectively.
Wasn't sure of how big it should be, seeing as harpies require males from other species in order to reproduce. Hopefully the size I choose is alright, though if it should be even smaller or vice versa, I'll change it.
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Said in stories to be taller than the tallest tree and with a wingspan large enough to blot out the sun when it flies overhead, the zizian harpy has been described as one of the largest known harpies in Felarya, even if its dimensions have been slightly exaggerated in these stories. Little is actually known about these harpies, though, as they've only been sighted around two or three times since the fall of Ur-Sagol, and it's generally believed that they nest elsewhere from Felarya, such as one of the continents across the sea.
From those that have been sighted, witnesses typically put their height at around 160 feet with a massive wingspan of proportional size. How they support such a great size while flying is unknown, though some speculate that they possess magic that nullifies their weight in a way akin to antigravity. Their physical traits have been described as condor-like, though with longer feathers and thicker wings. Wing coloration tends to be either grey or black with white markings on some of the feathers. Their speech patterns are unknown, due to the rarity of their sightings, though they are believed to be the same as those used by other harpy species.
Due to its extreme size, for a harpy at least, it is believed that only members of the larger races are potentially suitable "mates" for them. Those that are smaller may instead be viewed as food. Others that might be included on this dietary list potentially include young flying squid, small herds, and entire caravans. However, due to their previously mentioned rarity, the threat of being devoured by one of these aerial titans is extremely rare.