Some sources show him being born about 1173.
Some sources show death 07 March 1224/25.
William Longespee became Earl of Salisbury in right of his wife. In the beginning of King John's reign this nobleman was sheriff of Wiltshire, he was afterwards warden of the marches of Wales, and then sheriff of the counties of Cambridge and Huntingdon. About this period (14th John ) [1213], the baronial contest commencing, William Longespee at once espoused the royal cause and maintained it so stoutly that he was included by the barons amongst the evil councillors of the crown. The next year he was again constituted sheriff of Wilts and he held the office from that time during the remainder of his life. He had also a grant of the honour of Eye, in Suffolk, and was the same year a witness to the agreement made between King John and the barons as guarantee for the former. He was likewise a witness to the charter whereby John resigned his kingdom to the Pope. After this we find him a principal leader in the royal army until the very close of John's reign, when he swerved in his loyalty and joined, for a short period, the ranks of Lewis of France. Upon the accession, however, of Henry III [1216], he did homage to that monarch, particularly for the county of Somerset, which the king then gave him; and joining with William Marshall. governor of the king and kingdom, raised the siege of Lincoln when he was constituted sheriff of Lincolnshire and governor of Lincoln Castle, being invested at the same time with sheriff of the co. of Somerset, and governorship of the castle of Shirburne. His lordship soon afterwards accompanied the Earl of Chester to the Holy Land, and was at the battle of Damieta, in which the crescent triumphed. He served subsequently in the Gascon wars, whence returning to England, Dugdale relates, "there arose so great a tempest at sea that, despairing of life, he threw his money and rich apparel overboard. But when all hopes were passed, they discerned a mighty taper of wax burning bright at the prow of the ship and a beautiful woman standing by it who preserved it from wind and rain so that it gave a clear and bright lustre. Upon sight of which heavenly vision both himself and the mariners concluded of their future security, but everyone there being ignorant what this vision might portend except the earl, he, however, attributed it to the benignity of the blessed virgin by reason that, upon the day when he was honoured with the girdle of knighthood, he brought a taper to her altar to be lighted ever day at mass when the canonical hours used to be sung, and to the intent that, for this terrestrial light, he might enjoy that which is eternal." A rumour, however, reached England of the earls having been lost, and Hubert de Burgh, with the concurrence of the king, provided a suitor for his supposed widow, but the lady, in the interim, having received letters from her husband, rejected the suit with indignation. The earl soon after came to the king at Marlborough and, being received with great joy, he preferred a strong complaint against Hubert de Burgh, adding that, unless the king would do him right therein, he should vindicate himself otherwise to the disturbance of the public peace. Hubert, however, appeased his wrath with rich presents, and invited him to his table, where it is asserted that he was poisoned, for he retired to his castle of Salisbury in extreme illness and died almost immediately after, anno 1226. His lordship left issue, four sons and five daus., viz., William, his successor; Richard, a canon of Salisbury; Stephen, Justiciary of Ireland; Nicholas, bishop of Salisbury; Isabel, m. to William de Vesci; Ela, m. 1st, to Thomas, Earl of Warwick, and 2ndly to Philip Basset, of Hedendon; Idonea, m. to William de Beauchamp, Baron of Bedford; Lora, a nun at Lacock; and Ela, jun., m. to William de Odingsells. [Sir Bernard Burke, Dormant, Abeyant, Forfeited and Extinct Peerages, Burke's Peerage, Ltd., London, England, 1883, p. 167, d'Evereux, Earls of Salisbury]
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I have attached William to Rosamund Clifford as that is how it is shown by Brian Tompsett at Directory of Royal Genealogical Data, although he provides the following notes: "The House of Clifford, Chapter 5: Much controversy surrounds the identity of the Mother of William, for Rosamund was not the king's only mistress, though there are many who believe she was. Those who dispute Rosamund's claim base their case on the disparity in the ages of all concerned, but there is other evidence as well which can not be ignored. Unfortunately, the records date neither the birth of Rosamund nor that of her father, or her reputed sons. Documents also indicate an Ida, and an Ykenai as his mother. Died on Crusade."
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William Longsword, 3rd earl of Salisbury, Longsword also spelled LONGESP.E (d. March 7, 1226, Salisbury, Wiltshire, Eng.), an illegitimate son of Henry II of England, and a prominent baron, soldier, and administrator under John and Henry III. He acquired his lands and title from Richard I, who in 1196 gave him the hand of the heiress Ela, or Isabel, daughter of William, earl of Salisbury. He held numerous official positions in England under John.
He was sent on missions to France (1202) and to Germany (1209). In 1213-14 he organized John's Flemish allies, taking part in the destruction (1213) of the French fleet at Damme, then the port of Bruges, and leading the right wing of the allied army at Bouvines (July 27, 1214), where he was captured. He was exchanged and was back in England by May 1215, when he was employed by John in inspecting the defenses of royal castles and fighting the rebels in the southwest.
During John's war against the barons, Salisbury deserted the king after the landing of Louis of France (May 1216); he returned to royal allegiance, however, by March 1217, fought at Lincoln (May) and Sandwich (August), and attested the Treaty of Kingston (September 1217). Salisbury held various posts during the minority of Henry III and served against the Welsh in 1223 and in Gascony in 1225. He and his wife were benefactors of Salisbury Cathedral and laid foundation stones of the new cathedral in 1220. William was buried there and his effigy, a splendid early example, still survives. [Britannica CD '97]