The bride-to-be was not inconsequential. Henry chose Ermengarde de Beaumont, a great-granddaughter of Henry I. More immediately, she was the daughter of a viscount, Richard I of Beaumont-le-Vicomte. One chronicler, Walter Bower, described her as "an extraordinary woman, gifted with a charming and witty eloquence." She was 16 when she married William at Woodstock Palace.
William objected (uselessly), feeling that a king should have a bride of higher status, but Henry was feeling generous: he offered to pay all wedding expenses and to return to William the castles he had taken due to the Treaty of Falaise. The Castle of Edinburgh was considered Ermengarde's dowry.
William accepted the bride under these conditions (again, he had little choice), but he may not have accepted his "marital duties." He had fathered illegitimate children by at least two women, but children by Ermengarde did not come until at least seven years after the wedding. Perhaps it was only his concern to produce legitimate heirs that prompted him to finally treat Ermengarde as his wife.
Ermengarde was not just a footnote to William's reign, however. She supposedly helped a royal chaplain obtain a bishopric after she (and the king) accepted a bribe. In 1209, she mediated on behalf of her aging husband (he was 67) with King John. She is said to have taken over some of his duties in his later years, and went with William to England in 1212 to secure from King John the succession of their son Alexander.
After William's death in 1214, she was described as being distraught. She spent her final years founding the Cistercian Balmerino Abbey in Fife, where she was buried after her death in 1234.
So...what was the mediation in 1209 with King John, you ask? Well, despite previous failures, William never gave up his (and his ancestors') dream of expanding Scotland to include Northumbria. Let me tell you tomorrow how that went.
