Research Notes:
The Oxford Dictionary of National Biography states, [1]
Seagrave [Segrave], John, second Lord Seagrave (1256–1325), soldier and baron, was the eldest son of Nicholas Seagrave, first Lord Seagrave (1238?–1295), and his wife, Matilda, who was probably a Lucy. In 1270 John married Christiana, daughter of Hugh de Plessis, and granddaughter of John de Plessis, earl of Warwick.
… John Seagrave died some time before 4 October 1325. An account for funeral expenses was rendered by John Grey on 23 November 1325. He was buried at Chalcombe Priory. Christiana survived him, and was still alive in 1331. His brother Nicholas Seagrave was also summoned as a peer from 1295 until his death in 1321, while another brother, Gilbert Seagrave (d. 1316), was bishop of London. John's eldest son, Stephen, who became the third baron, married Alice Arundel, but died some time before 12 December 1325, and was similarly buried at Chalcombe. His younger son and namesake married Juliana, daughter and heir of John Sandwich. The title was inherited by Stephen's ten-year-old son, John, whose wardship and marriage were entrusted to Thomas of Brotherton, earl of Norfolk. John in due course married Margaret Brotherton, the earl's daughter and sole heir, consolidating the family's status and territorial position, but died in 1353, leaving a daughter, Elizabeth (b. 1338), who at the age of eleven married John (III) Mowbray (d. 1368).
[1] A. J. Musson, "Seagrave [Segrave], John, second Lord Seagrave (1256-1325)," Oxford Dictionary of National Biography (Oxford University Press, 2004, online edition), [Oxford_Dictionary_National_Biography], [OxfordDNB(UM)].