Original sentence: Beorma would have been the founder or ancestor of a tribe long before its arrival in what was to become Anglo-Saxon Mercia, the head of a tribe or clan of kinsmen who travelled together for the purpose of migration (and who settled in Mercia), or the leader of a (possibly mercenary) group with whom he shared a contractual obligation (the frankenpledge)
Edited sentence: Beorma would have been the founder or ancestor of a tribe long before its arrival in what was to become Anglo-Saxon Mercia, the head of a tribe or clan of kinsmen who travelled together for the purpose of migration (and who settled in Mercia), or the leader of a (possibly mercenary) group with whom he shared a contractual obligation (the frankpledge)