Though he was not a member of the House of York, Edmund Mortimer, 5th and last Earl of March had a huge impact on it, not least by getting all three of Edmund of Langley's children into trouble on his behalf, and then by conveniently dying and leaving all he had to his sister's son, Richard, Duke of York.
Note to Shakespeare lovers - do not confuse this Edmund with his Uncle Edmund, who died in 1409. The Bard of Avon tends to do this, but you see, he was not a historian.
Born on 6 November 1391, Edmund was the son of Roger Mortimer, Earl of March and Alianore Holland, eldest daughter of the Earl of Kent (and half-niece to Richard II). He was only six years old when his father was killed in Ireland in July 1398 and the custody of his estates shared out between Edward of York, Duke of Aumale, John Holland, Duke of Exeter, and Thomas Holland, Duke of Surrey - his mother's brother.
After the accession of Henry IV Edmund and his brother Roger became what Pugh, with his characteristic bluntness, calls 'in effect, state prisoners'. They were lodged at Berkhampstead and Windsor Castles until Constance of York's 1405 plot, after which they were moved to Pevensey for the next three years.
In 1409 they were transferred to the household of Henry, Prince of Wales (future Henry V), and in November 1412 Edmund was given livery of his estates, his brother Roger dying soon afterwards. However Edmund wanted to choose his own wife - Anne Stafford - and because of this marriage Henry V fined him the unprecedented sum of 10,000 marks. (About £6,666.)
To be clear, Henry was entitled to levy a fine, but the amount was wholly unreasonable. Not even Henry VII matched this sort of greed. Because of this, and the cost of equipping himself to join Henry's French expedition, Edmund had to raise a huge loan, mortgaging a large proportion of his English and Welsh estates to a syndicate of rich individuals. He still owed much of the money at the time of his death.
It seems as certain as anything can be that this was the motive for his agreement to join in the Southampton Plot organised by his former brother-in-law, Richard of Conisbrough, Earl of Cambridge. However his nerve quickly cracked and it appears he was the one who betrayed the plotters, receiving a pardon on 7 August 1415.
The result of course was that no one trusted him with any further plots, and his 'threat' to Henry V was effectively neutralised. Though absent from Agincourt due to sickness he took a significant part in Henry's French wars, not least because Henry didn't care to leave him in England. His efforts received no reward.
After Henry V's death, Edmund was accused of having too large a household, and of keeping open house to win support. His kinsman Sir John Mortimer was accused of treason and, after escaping from the Tower (twice!) was executed. The final solution to the Mortimer question was to send Edmund off to Ireland as Lieutenant in 1424. Like his father before him, he died there. In January 1425. He had no children, though his widow went on to have children with another man.
Edmund's brother and sisters had died before him (the Mortimers rarely seem to have made old bones) and only Anne, Richard of Conisbrough's wife, had had any children. So the Mortimer inheritance came to the House of York, and, particularly after the debts had been paid and Earl Edmund's widow had died, completely transformed the family fortunes. Richard, Duke of York, was to be the richest subject since Henry Bolingbroke - and with similar results.
Main source for this again the invaluable Henry V and the Southampton Plot by T.B. Pugh.