"As for the Harii, quite apart from their strength, which exceeds that of the other tribes I have just listed, they pander to their innate savagery by skill and timing: with black shields and painted bodies, they choose dark nights to fight, and by means of terror and shadow of a ghostly army they cause panic, since no enemy can bear a sight so unexpected and hellish; in every battle the eyes are the first to be conquered." ~Tacitus in Germania, c 98 AD.
The Harii were merely one subgroup amid a large confederation of tribes of Germanic Peoples that lived in central Europe between 100 BC and 300 AD. Known to have a hierarchical society, often with leaders elected to kingship by all free men, they practiced both agriculture and animal husbandry with a technological level typical of the protohistoric period. They dwelt in the region then known to the Romans as Magna Germania, which comprised almost all of the northern part of central and eastern Europe.
Regarding those Romans, by the way, the Germanic peoples did not always get along with them. Though sometimes they established trade relations, the Germanic tribes were often at war with Rome. They did not have armor or weapons supplies anywhere approaching the abundance of the Romans', with only the most important leaders even wearing helmets into battle, and also lacked standing armies, with their approach of warfare being raids consisting of only 10 to 1000 warriors. This did not mean that they deserved to be underestimated, as even the potent Roman legions considered their hostile presence to be a serious threat; an outstanding example being the Battle of the Teutoburg Forest, where a vast force of barbarian warriors ambushed and slaughtered three entire Roman legions, along with their auxiliaries, in 9 AD.
The Harii were a subgroup of tribes known as the Lugii, whom Strabo described in the first century as "a great people," and who lived between the Viadua (Oder) and Vistula rivers in the 1st to 4th centuries AD. The Lugii were known to the Roman writer Tacitus, who mentions that they were divided into many tribes, and in his work Germania, he mentions the five most powerful: Harii, Manimi, Helveconae, Helisii and Nahanarvali.
The Harii he mentions as being particularly fearsome, denoting their fierce skill at arms that was cleverly augmented through psychological warfare designed to terrify their enemies: they would paint their shields and bodies black, and attack in the middle of the darkest nights to sow fear and break the will of their enemies to resist.
Though most historians consider the Lugii to have been absorbed into the Vandals by the 3rd century, they are still remembered today for the dread they evoked on the battlefield of those who dared to face them, and for their descendants, who would one day sack Rome.
In the Timeline of 1337-A, the Harii are blamed for a horrifying attack on Legion XI of the Roman Empire that obliterated their camp, killed all soldiers present, slew their general, and, in the ultimate disgrace--looted the legionary standard.
This is was in fact due to the actions of Sakura Xadium Aino, her sentient blade Arataka, Catriona, and Daniel Kakaku, and their encounter with a mysterious time active whose collective actions that night doomed the Romans. As a result of the night's events, in the 1337-A timeline the Harii are ascribed even greater prowess and infamy by historians due to this apparent attack that was seemingly able to totally overwhelm a fortified legion. The supposed fact that Germanic barbarians in the year 116 were able to disgrace and nearly annihilate the XI Legion in a "Teutoburg Revisited" is yet another error in the (substantially-misinformed) history books of 1337-A.