Mélusine is said to have founded the cities of:
Lusignan, Parthenay, Tiffauges and Talmont (France)
The legend also says she built the walls of La Rochelle, and several churches (St-Paul-en-Gâtine, Partenay, Niort...).
According to the legend, she founded the Lusignan family (a powerful mansion of the middle ages). She is also known as "La mère Lusigne" (mother Lusigne), founder of Lusignan.
Her story (roughly, from memory):
Her legend starts in Scotland. Daughter of Persine and Elinas, king of Scotland, she is in exile in the island of Avalon when she decides to lock her father in a mountain (with the help of her two younger sisters), as a revenge for their exile. Her mother, fairy Persine punishes her three daughters, and especially Mélusine, since she's the oldest. She condemns her to look like a normal woman from head to waist, and a snake from waist to feet on Saturdays. If she finds a man who'd accept not to see her on Saturdays, she would live a normal woman life, and have a long beautiful lineage. If not, she'd be tormented forever.
She meets Raymondin (of Lusignan) who accepts to marry her. They have 10 children, each who would accomplish great things later. She builds castles, churches and Raymondin becomes very powerful. But his brother, who's jealous, convinces him to go and see his wife on a saturday. She leaves (flies off the window). Sometimes at night, she came to visit her children. The legend says that each time a descendant of the Lusignans dies, people can hear Mélusine cry.