William de Forz, comte d'Aumale, Surety of the Magna Carta - Who is this person? William de Forz, comte d'Aumale, Surety of the Magna Carta

That would be this fellow:

WILLIAM de Forz, son of GUILLAUME de Forz Comte d’Aumâle & his wife Hawise Ctss d’Aumâle (-29 Mar 1241). The 13th century Histoire des ducs de Normandie et des rois d’Angleterre records that "Aubemalle…Havi le contesse" married successively "le conte de Mandeville…Guillaumes" by whom she was childless, "Guillaume de Fors" by whom she had "i fills…Guillaumes", and thirdly "Bauduin de Biethune"[983]. The Cronicon Cumbriæ names “Hawysiam” as the child of “Willielmus Grossus comes Albemarliæ” and his wife, adding that he was succeeded by “Willielmus de Fortibus comes Albemarliæ”, in turn succeeded by “alter Willielmus de Fortibus”, and the latter by “Avelina” who married “Edmondo fratri domini Regis E” and died childless[984]. Lord of Holderness. He succeeded as titular Comte d'Aumâle. “Willielmus comes de Albamare filius Hawisæ comitissæ de Albamare” donated property to Garendon Abbey by undated charter[985]. He was one of the 25 conservators of Magna Carta[986]. Matthew of Paris records that he died in the Mediterranean[987].

m (1214) AVELINE de Montfichet, daughter of RICHARD de Montfichet, of Stansted, Essex & his wife Milicent --- (-[Nov] 1239, bur Thornton Abbey). A manuscript history of the foundation of Melsa Abbey records that “Willielmum de Forz secundum” married “Avelinam…filiam…Ricardi de Munfichet”[988]. Matthew of Paris records the death in 1239 of "Aveline de Forz Ctss d'Aumâle"[989]. An undated writ "52 Hen III", after the death of "Richard de Muntfichet", records that "he had 3 sisters, Margery, Avelina and Philippa…from Avelina issued William de Fortibus earl of Albemarle whose heirs are under age and in the king’s wardship"[990]. A writ dated 20 Feb "3 Edw I", after the death of "Avelina late the wife of Edmund the king’s brother", names as heirs of "Richard de Munfichet…his three sisters…the second sister Avelina married William de Forz earl of Albemarle, and from them issued William de Forz the last earl of Albemarle, who had two sons Thomas and William who died without heirs of their bodies…" and the declaring the descendants of the sisters of Aveline de Montfichet as heirs of Aveline de Forz[991]. https://fmg.ac/Projects/MedLands/normacre.htm#WilliamForzdied1241A

("CP" in the citations refers to Cokayne's Complete Peerage, q.v. under "Albemarle")
https://archive.org/details/completepeerage01cokahrish/page/n77/mod...

A couple of complications are that he held a French title, not an English one (so strictly speaking was a Count, not an Earl), and he changed sides, first siding with the Barons, then with King John, then as he saw his advantage. Sneaky of him!

Here's another source, with much more detail: https://magnacarta800th.com/schools/biographies/the-25-barons-of-ma...

Per various sources he had one (1) son, another William, but all of his grandchildren died young and/or without heirs. The subsequent history of the title is, shall we say, confusing:
https://en.wikipedia.org/wiki/Counts_and_dukes_of_Aumale