Two entries with the name Nascien.

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Nascien

Nasciens

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A descendant of Nascien d’Orberique, the son of Narpus, an ancestor of Galahad. Father of Alain the Large.

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Sources
Vulgate Queste del Saint Graal | 1215-1230
Vulgate Estoire del Saint Graal | 1220-1235
Le Morte Darthur | Sir Thomas Malory, 1469-1470

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Nascien

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In Welsh legend, the son of the King of Denmark, and one of Arthur’s Three Royal Knights. He is described as wise, handsome, and skilled at arms. Likely, Welsh storytellers simply borrowed the name from the Grail stories.

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Source
Triads of the Island of Britain (Welsh ”Triads”) | 11th century to 14th century

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