The chief original sources for John's life are Froissart, the maliciously hostile Chronicon Angliae (1328-1388), and the eulogistic Chronicle of Henry Knighton (both the latter in the Rolls Series).
In 1402 he routed the forces of the Mortimers at Bryn Glas near Knighton in Maesyfed, where he captured Sir Edmund Mortimer, the uncle and guardian of the legitimate heir to the English throne, the young earl of March.
The rebel achieved his greatest success in June 1402, when he surprised and routed the whole levy of the marcher lords at Bryn Glas, between Pilleth and Knighton, capturing (among many other prisoners) Sir Edmund Mortimer, the uncle and guardian of the young earl of March, whom all malcontents regarded as the rightful monarch of England.
In the i4th century there is a significant deterioration in the monastic chroniclers, and their place is taken by the works of secular clergy like Adam Murimuth, Geoffrey the Baker, Robert of Avesbury, Henry Knighton and the anonymous author of the Eulogium historictrum.
(London, 1869); Chronicon of Henry Knighton, ed.