Great Conspiracy The Great Conspiracy is a term given to a year - long war that occurred in Roman Britain near the end of the Roman occupation of the island . The historian Ammianus Marcellinus described it as a " barbarica conspiratio " that capitalized on a depleted military force in the province brought about by Magnentius ' losses of the Battle of Mursa Major after his unsuccessful bid to become emperor . The conspiracy . In the winter of 367 , the Roman garrison on Hadrian ' s Wall rebelled , and allowed Picts from Caledonia to enter Britannia . Simultaneously , Attacotti , the Scotti from Hibernia , and Saxons from Germania landed in coordinated and pre - arranged waves on the island ' s mid - western and south - eastern borders , respectively . Franks and Saxons also landed in northern Gaul . These warbands managed to overwhelm nearly all of the loyal Roman outposts and settlements . The entire western and northern areas of Britannia were overwhelmed , the cities sacked and the civilian Romano - British murdered , raped , or enslaved . Nectaridus , the Count of the Saxon Shore , was killed and the Dux Britanniarum , Fullofaudes , was either besieged or captured , the remaining loyal army units staying garrisoned inside southeastern cities . The miles " arcani " or local Roman agents that provided intelligence on barbarian movements seem to have betrayed their paymasters for bribes , making the attacks completely unexpected . Deserting soldiers and escaped slaves roamed the countryside and turned to robbery to support themselves . Although the chaos was widespread and initially concerted , the aims of the rebels were simply personal enrichment and they worked as small bands rather than larger armies . Roman response . Emperor Valentinian I was campaigning against the Alamanni at the time and unable to respond personally . A series of commanders to act in his stead were chosen but swiftly recalled . The first was Severus , the emperor ' s comes domesticorum , soon replaced by Jovinus , the magister equitum ; rumours of disasters dogged them , however , and it took almost 15 months before a capable replacement was sent . In the spring of 368 , a relief force commanded by Count Theodosius arrived in Britannia from Gaul . He brought with him four units , " Batavi " , " Heruli " , " Iovii " and " Victores " as well as his son , the later Emperor Theodosius I and probably the later usurper Magnus Maximus . He marched from Richborough to Londinium and began to deal with the invaders . An amnesty was promised to deserters which enabled Theodosius to regarrison abandoned forts . A new Dux Britanniarum was appointed , Dulcitius , with Civilis granted vicarius status to head a new civilian administration . By the end of the year , the barbarians had been driven back to their homelands ; the mutineers had been executed ; Hadrian ' s Wall was retaken ; and order returned to the diocese . Considerable reorganization was undertaken in Britain , including the creation of a new province named Valentia , probably to better address the state of the far north . Claudian suggests that naval activity took place in northern Britain . It is possible that Theodosius mounted punitive expeditions against the barbarians and extracted terms from them . Certainly , the Notitia Dignitatum later records four units of Attacotti serving Rome on the continent . The " areani " were removed from duty and the frontiers refortified with co - operation from border tribes such as the Votadini , marking the career of men such as Paternus . Political effects . Theodosius returned to Rome a hero , and was made senior military advisor to Valentinian I . A decade later , his son became emperor . The Romans were able to end much of the chaos , though raids by all of the people listed above did continue . Fictional references . Fictional accounts of the Great Conspiracy were featured in Wallace Breem ' s historical novel " Eagle in the Snow " , Stephen R . Lawhead ' s fantasy novel " Taliesin " , and Jack Whyte ' s historical novel , " The Skystone " .