Creiddylad is from the Welsh Creiddylad (jewel). Creiddylad is the daughter of Lludd Llaw Eraint in the Mabinogion — the collection of medieval Welsh prose tales (12th-13th c.). Central to Culhwch and Olwen (c. 1100), one of the earliest Arthurian texts. She is the prize fought over by Gwythyr ap Greidawl and Gwyn ap Nudd every May 1st (Calan Mai), representing the seasonal contest between summer and winter. Shakespeare's Cordelia in King Lear is widely considered a anglicization derived from Creiddylad.
Creiddylad does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
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In Pythagorean numerology the letters of Creiddylad reduce to 4, The Builder. This is a traditional interpretive system, not a factual claim about the name.