Though the Danes had been attacking the British Isles since an 835 raid on Kent, the warriors had not stayed the winter. It was only in 865, with the Great Army, that they stayed in their conquered territory, establishing the Danelaw. Guthrum had been the de facto king of Denmark since committing regicide in 854.
Though East Anglia, Northumbria and Mercia are traditionally perceived to have been in the heptarchy they were largely independent kingdoms until its conquest by the Danes in 869 and 867, respectively - where Mercia was conquered by Wessex in 874. All other members of the heptarchy had been conquered by Wessex and by this time had no form or king.
