Simply brand will do.
As in "He grabbed a brand from the fire and thrust into the face of the undead.."
In fact it is an interesting one. Historically speaking "brand" means burning Oxford Dictionaries : Brand from the Old English brand 'burning' related to German "Brand".
From here you get a Brand of wood from a fire, then to Branding Iron, then to the mark made by a Branding Iron - a Brand, from there to a mark in general - in particular a trade mark or a Brand.
I am glad you made me look that one up.