“Pedigree”–“Sir Isumbras” appears in the same Thornton Manuscript which features “Sir Eglamour”, in Middle English. Wyllyam Copland printed another edition, but the surviving copy of this printing has no date.
This is a tale with medieval morals. Isumbras is punished for his pride by losing his belongings, and then his three sons (carried off by a lion, a leopard, and a unicorn), his wife (carried off by a Saracen soudan), and his mantle (to an eagle). Isumbras takes up skilled labor, until a battle gives him a chance to turn warrior again. After seven (more?) years of wandering in the Holy Land, God forgives Isumbras, gradually reunites him with his family and (presumably) lends Divine Providence to father and (now grown) sons in “winning back” several kingdoms for Christendom–and for Isumbras and his sons to rule.
Google books copy of “Thornton Romances” here: