Gormond/Gormont/Gormund et Isembart/Isembard (Gormund and Isembard). second half of Eleventh/first half of Twelfth Century AD (AD 1068/1150):

Bayot, Alphonse, ed. Gormont et Isembart: Fragment de Chanson de Geste du XIIe Siècle. Paris: Librairie Ancienne Honoré Champion, 1914.
https://ia601604.us.archive.org/26/items/gormontetisembar00bayouoft/gormontetisembar00bayouoft.pdf

Fundenburg, George Baer. Feudal France in the French Epic: A Study of Feudal French Institutions in History and Poetry, pp17, 24, 26, 69, 70, 71, 72, 77, 92, 100, 102, 107, 114, 115, 120. Princeton, New Jersey: Columbia University, 1918.
https://ia800205.us.archive.org/25/items/cu31924027204183/cu31924027204183.pdf

Menichetti, Caterina, and Maria Teresa Rachetta. “The Gormond et Isembart Legend in the French Monastic Chronicles of the 13th Century”
in Ailes, M Arianne J, Philip E Bennet, and Anne E Lizabeth Cobby, eds. Epic Connections/Rencontres Épiques: Proceedings of the Nineteenth International Conference of the Société Rencesvals, pp465-493. Edinburgh: University of Edinburgh, through the Societe Rencesvals British Branch, 1 March 2015.
https://www.academia.edu/12729389/The_Gormond_et_Isembart_Legend_in_the_French_monastic_chronicles_of_the_13th_century_in_Epic_Connections_Rencontres_épiques_Proceedings_of_the_Nineteenth_International_Conference_of_the_Société_Rencesvals_ed_by_M_J_Ailes_et_al_2_voll_Edinburgh_2015_II_pp_465_493