The Assault on Grantebridge was a Viking assault in 873 on the city of Grantebridge by the Great Summer Army led by Soma against the occupying Anglo-Saxons led by ealdorman and Order of the Ancients member Wigmund.
Some years prior to 873, the Great Summer Army under jarl Guthrum captured Grantebridge from the Kingdom of Mercia. When Guthrum headed south to combat against the Kingdom of Wessex, he appointed his close aide Soma as the jarlskona of the settlement. Under her rule, Grantebridge was developed from a minor settlement to a prospering port town. With help from her closest advisors Lif, Birna and Galinn, Soma built a tunnel beneath the longhouse as an intended emergency exit. It was considered a hidden secret that only Soma and her advisors would know.
Meanwhile, the Anglo-Saxons made repeated attempts to recapture the city, one of which was led by the ealdorman Wigmund, who was also secret a member of the Order of the Ancients. However, the attempts failed each time and the Vikings remained in control of the city up till the early 870s.
Ultimately, Soma's advisor Galinn chose to betray his clan and the city to the Saxons in the belief of following his "destiny". He showed the secret tunnel entrance to Wigmund, and devised a plan of his own escape during the Saxons attack on the city. During the night of the attack, the Saxons entered and attacked through tunnel, while Galinn escaped with a bucket of yellow paint stolen from Lif, and hurriedly painted his longship's mast yellow such that Wigmund's men would know and not attack his ship.
Soma and the Vikings were surprised by the attack, and were ultimately driven out. While she managed to seek temporary shelter at the village of Middeltun, the rest of her men were scattered throughout the Fenlands and the Roman ruin of Duroliponte in Grantebridgescire. With aid from Eivor Varinsdottir of the Raven Clan, she gathered her men and advisors and planned an attack to recapture the city.
With her army regrouped, Soma and his men made a stand outside Grantebridge with Eivor as an ally. Soma called out Wigmund to negotiate as a guise while her men secretly prepared to destroy the city gates with explosive jars. When Wigmund refused to surrender the city to Soma, the Vikings launched the attack, with Eivor destroying the city gates with the explosive jars. The Vikings fought their way through the walls and reached the inner wall containing the longhouse and the market.
Although expecting Wigmund to be hiding in the longhouse, his lieutenant appeared instead and confronted the Vikings outside. Revealing that Wigmund has escaped, the lieutenant and the remaining Saxons tried to put up a fight but were ultimately defeated and killed. As a gesture for helping to turn the tide and recapture the city, Soma requested Eivor to blow the sound horn to signal their victory.
In the aftermath of the battle, the Vikings held a feast to celebrate their victory, with Eivor in attendance. The next day, Eivor met with Soma within the longhouse, and was shown the secret tunnel that Soma had built with her advisors. Aware that the clan were betrayed by one of her advisors, Soma pleaded with Eivor to help her investigate and uncover the traitor, given her neutrality as an outsider.