Quote: "Just turn the regular texture greyscale, up the contrast, and adjust the brightness."
Thats just going to illuminate the whole object if used as an illumination map.
An illumination map simply is a black map which has white areas, these white areas will appear with full brightness in game and are not affected by lighting or ambience.
I dont have the medieval pack but whoever made it, its likely they included the map with the model, look for a ****_I.dds texture in the entity's folder if its there simply apply the illumination shader to the model.