Encyclopedia of Myth in A Song of Ice and Fire
Chapter I: House Wynch of Iron Holt
House Wynch – a noble house of The Iron Islands, from Iron Holt on the isle of Pyke, their current lord is Waldon Wynch, their sigil is A bloody moon on purple. When Euron Greyjoy returns to claim the crown, Lord Wynch is the first to bend his knee. For this he is rewarded with half of the Botley lands.
Mythical Astronomy on House Wynch:
Now, where were we… bloodstone, bloody stones in the water, bloody tides from sacrificed moons… got it. Here’s one more little fun tidbit regarding bloody moons and sacrifice. On the Iron Islands, we find House Wynch, whose sigil is a bloody crescent moon on a field of purple. A winch is a thing which pulls heavy objects out of place – we’re going to need a very big winch for the moon, of course, but that’s another essay. And that sigil – it’s a crescent moon which is literally in a bed of blood. Said another way, a moon crescent could be seen as a blade made of moon – flaming sword moon meteors, in other words, the moon stones which were covered in blood, like the crescent moon of House Wynch.
House Wynch really does not do anything important in any of the novels, nor even in Ironborn history. Literally the only noteworthy thing that George has written about them is their sigil – and I suspect that’s because their sigil IS the important thing about them. A bloody crescent moon is easy to understand, given what we’ve just looked at concerning sickles and crescent moons and blood sacrifice, and attached to the word winch, it speaks of pulling down the moon. The purple background may be meant to remind us of the Amethyst Empress and Daenerys, the purple-eyed moon maidens.
(The Bloodstone Compendium III: Waves of Night and Moon Blood)
In the east sat an old woman in Iron-woodand nurtured there offspring of Fenrira certain one of them in monstrous formwill be the snatcher of the moon
A witch dwells to the east of Midgard, in the forest called Ironwood: in that wood dwell the troll-women, who are known as Ironwood-Women [járnviðjur]. The old witch bears many giants for sons, and all in the shape of wolves; and from this source are these wolves sprung. The saying runs thus: from this race shall come one that shall be mightiest of all, he that is named Moon-Hound [Mánagarmr]; he shall be filled with the flesh of all those men that die, and he shall swallow the moon
Theon was more interested in ships than gods. Among the masts of countless fishing boats, he spied a Tyroshi trading galley off-loading beside a lumbering Ibbenese cog with her black-tarred hull. A great number of longships, fifty or sixty at the least, stood out to sea or lay beached on the pebbled shore to the north. Some of the sails bore devices from the other islands; the blood moon of Wynch, Lord Goodbrother’s banded black warhorn, Harlaw’s silver scythe. Theon searched for his uncle Euron’s Silence. Of that lean and terrible red ship he saw no sign, but his father’s Great Kraken was there, her bow ornamented with a grey iron ram in the shape of its namesake.
Dark clouds ran before the wind as the first light stole into the world. The black sky went grey as slate; the black sea turned grey-green; the black mountains of Great Wyk across the bay put on the blue-green hues of soldier pines. As color stole back into the world, a hundred banners lifted and began to flap. Aeron beheld the silver fish of Botley, the bloody moon of Wynch, the dark green trees of Orkwood. He saw warhorns and leviathans and scythes, and everywhere the krakens great and golden. Beneath them, thralls and salt wives begin to move about, stirring coals into new life and gutting fish for the captains and the kings to break their fasts. The dawnlight touched the stony strand, and he watched men wake from sleep, throwing aside their sealskin blankets as they called for their first horn of ale. Drink deep, he thought, for we have god’s work to do today.(A Feast for Crows, The Drowned Man)
A crowd had gathered round to wish him well and seek his favor. Victarion saw men from every isle: Blacktydes, Tawneys, Orkwoods, Stonetrees, Wynches, and many more. The Goodbrothers of Old Wyk, the Goodbrothers of Great Wyk, and the Goodbrothers of Orkmont all had come. The Codds were there, though every decent man despised them. Humble Shepherds, Weavers, and Netleys rubbed shoulders with men from Houses ancient and proud; even humble Humbles, the blood of thralls and salt wives. A Volmark clapped Victarion on the back; two Sparrs pressed a wineskin into his hands. He drank deep, wiped his mouth, and let them bear him off to their cookfires, to listen to their talk of war and crowns and plunder, and the glory and the freedom of his reign.(A Feast for Crows, The Iron Captain)
So, it’s certainly possible (and strongly suggested) that the Greenseers were responsible for destroying the Second Moon of Planetos.