Looks like he participate on the wars against
Arminius. During that time he met a female druid that was acting as a double spy and she left him that scar he has on his face, that actually is a curse that make him lost an important battle. This would have cost him his career but
Germanicus took care of him till he died. After Germanicus death, Longinus met Jesus on the crucifixion. Also, seems that female druid was a mage that has been living for a long time without aging and serve under
Vercingetorix and of course Erice had to point out how that case is similar to Chitose's