Depiction of Beauséant, the "sacred banner" (or gonfanon) of the Templars, argent a chief sable.
"the Knights Templar [...] carried  white shields with red crosses but [their] sacred banner, Beauséant, was white with a black chief"
"Flags and standards", Colum Hourihane (ed.), The Grove Encyclopedia of Medieval Art and Architecture, Volume 1 (2012), p. 514.

D. H. Wolf, Internationales Templerlexikon (2015), 130f.
Pierre Palliot, LA VRAYE ET PARFAITE SCIENCE DES ARMOIRIES, OV L'INDICE ARMORIAL DE FEV MAISTRE LOVVAN GELIOT (1660), p. 77.
Own work
Own work