General Notes

Sir William Thomas, called "Y Marchog Glas o Went" ("The Blue Knight of Gwent"), of Raglan, Mon, the (feudal) Lordship of which he bought from 1st Lord (Baron) Berkeley; knighted 1415; married 1st his former employer Elizabeth (to whom he had been steward), daughter and heiress of Sir John Bluet, of Raglan, and widow of Sir James Berkeley (by whom she was mother of the 1st Lord (Baron) Berkeley; married 2nd, as her 2nd husband, Gwladus (died 1454), daughter of Dafydd Gam (killed at Agincourt) and widow of Sir Roger Vaughan, of Tretower (also killed at Agincourt), and died 1446.  [Burke's Peerage]

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Built the "Great Tower" of Raglan Castle.

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Copied from Herbert, George biography, 88.1911 encyclopedia.org/H/HERBERT_GEORGE.htm:
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Thomasís fifth son, William or Gwilim ap Thomas, who died abt 1446, was the first man of the family to make any figure in history. This Gwilim ap Thomas was steward of the lordships of Usk and Caerleon under Richard, duke of York. Legend makes him a knight on the field of Agincourt, but his knighthood belongs to the year 1426. He appears to have married twice, his first wife being Elizabeth Bluet of Raglan, widow of Sir James Berkeley, and his second a daughter of David Gam, a valiant Welsh squire slain at Agincourt. Royal favour enriched Sir William, and he was able to buy Raglan Castle from the Lord Berkeley, his first wifeís son, the deed, which remains among the Beaufort muniments, refuting the pedigree-makerís statement that he inherited the castle as heir of his mother Maude daughter of Sir John Morley.

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Note: Apparently there are two different stories about how William Herbert ended up with Raglan Castle.