Supposed to live in caverns, under the beach and under the sea, the groac'h has power over the forces of nature and can change its shape.

It is mainly known as a malevolent figure, largely because of Émile Souvestre's story La Groac'h de l'île du Lok, in which the fairy seduces men, changes them into fish and serves them as meals to her guests, on one of the Glénan Islands.

Several place-names of Lower Brittany are connected with the groac'h, especially the names of some megaliths in Côtes-d'Armor, as well as the island of Groix in Morbihan and the lighthouse of La Vieille.

The origin of those fairies that belong to the archetype of "the crone" is to be found in the ancient female divinities demonized by Christianity.

The groac'h has several times appeared in recent literary works, such as Nicolas Bréhal's La Pâleur et le Sang (1983).

[8] Joseph Mahé [fr] speaks (1825) of a malicious creature that he was frightened of as a child, reputed to inhabit wells in which it drowned those children that fell in.

[12] One of them is said to frequent the neighborhood of Kerodi, but the descriptions vary: an old woman bent and leaning on a crutch, or a richly dressed princess, accompanied by korrigans.

[2] The storyteller Pierre Dubois describes them as shapeshifters capable of taking on the most flattering or the most repugnant appearance: swans or wrinkled, peering hobgoblins.

[15] André-François Ruaud [fr] relates it rather to undines,[16] Richard Ely and Amélie Tsaag Valren to witches,[17] Édouard Brasey describes it as a "lake fairy".

[20] The groac'h of Lanascol Castle could shake the dead autumn leaves and turn them to gold, or bend the trees and make the ponds ripple as it passed.

Most often, the groagez are described as being solitary in their retreats under the sea, in a rock or in the sands,[3] but some stories tell of an entirely female family life.

[26] A story collected by Anatole Le Braz makes one of these fairies the personification of the plague: an old man from Plestin finds a groac'h who asks for his help in crossing a river.

Houarn goes to the island of Lok and gets into an enchanted boat in the shape of a swan, which takes him underwater to the home of the groac'h.

He accepts, but when he sees the groac'h catch and fry fish which moan in the pan he begins to be afraid and regrets his decision.

Houarn tries to escape but the groac'h comes back and throws at him the steel net she wears on her belt, which turns him into a frog.

At the top of a rock, Bellah finds a little black korandon, the groac'h's husband, and he tells her of the fairy's vulnerable point.

She goes to the groac'h, who is very happy to receive such a beautiful boy and yields to the request of Bellah, who would like to catch her fish with the steel net.

The metamorphosed men and the korandon are saved, and Bellah and Houarn take the treasures of the fairy, marry and live happily ever after.

[19] According to this tale, collected by G. Le Calvez at the end of the 19th century, a vor Groac'h, "sea fairy",[38] lives in a hollow rock on the island.

[39] This story was collected by Anatole Le Braz, who makes reference to the belief in fairies among people of his acquaintance living near his friend Walter Evans-Wentz.

[41] Pierre Saintyves cites from the same commune a "table of the old woman", a dolmen called daul ar groac'h.

[42] At Maël-Pestivien three stones two meters high placed next to each other in the village of Kermorvan, are known by the name of Ty-ar-Groac'h, or "the house of the fairy".

[47] According to Souvestre and the Celtomaniac Alfred Fouquet [fr] (1853), the island of Groix got its name (in Breton) from the groagez, described by them as "druidesses" now seen as old women or witches.

[48] For the writer Claire de Marnier this tradition, which makes the islanders sons of witches, is a "remarkable belief" peculiar to "the Breton soul".

[51] Similarly, Anatole Le Braz cites Barr-ann-Heol near Penvénan, as a dangerous place where a groac'h keeps watch, ready to seize benighted travellers at a crossroads.

According to Marc Gontard, the groac'h demonstrates the demonization of ancient goddesses under the influence of Christianity: it was changed to a witch just as other divinities became lost girls and mermaids.

[58] Pierre Dubois likens the groac'h to many maleficent water-fairies, like Peg Powler, Jenny Greenteeth, the mère Engueule and the green ogresses of Cosges, who drag people underwater to devour them.

[61] Edain McCoy equates the groac'h with la Vieille, citing especially the regular translation of the word as "witch".

[63] A "mystical and fantastic" novel,[64] La Pâleur et le Sang includes the groac'h among the mysterious and almost diabolical forces that assail the island of Vindilis.

[65] In Jean Teulé's novel Fleur de tonnerre (2013), groac'h is a nickname given to Hélène Jégado when she is a little girl, in Plouhinec.