A niece of Arthur, about whom Guenevere gosspied unjustly.
As a result of this, Laneth brought a magic mantle to the court to test the fidelity of the women present. All but one failed and Guenevere herself wast the most notable failure.
See also
Chastity test | The Legend of King Arthur
Source
”Lanethen Mantel” | 15th century