Complex sentence: Along with other forces, the Great Heathen Army then invaded Wessex, perhaps in December 870 (within a few weeks of killing Edmund) or after having spent a year pillaging and consolidating their position in East Anglia, before proceeding to attack Mercia and Northumbria.
Split into simpler sentences: Along with other forces, the Great Heathen Army then invaded Wessex, perhaps in December 870, which would have been within a few weeks after Edmund's death. Alternatively the Danes may have spent a year pillaging and consolidating their position in East Anglia before proceeding to attack Mercia and Northumbria.