The Germania book, Written by Cornelious Tacitus, is one of the only sources of information regarding the Harri warriors that attacked and defeated the Great Roman Army. Tacitus describes the attacking warriors as being painted completely black, having black shields and weapons. The only things you could see from the attacking forces were the whites of their eyes as they attacked. While the information written down from Tacitus is an account from surviving soldiers, it is believed to be accurate.
The Harri Warriors of Heilung are based off of this depiction. They demonstrate the effect that the Harri could have had on an opposing force.
Here is a link to the Germainia and Agricola