Short answers, pulled from the story.
Charlemagne's first war against the Saxons began in January 772 following a Saxon expedition that set fire to the church of Deventer. This event served as the casus belli for the Frankish king entering Saxon territory with the intent to conquer.
Widukind led the Saxon free tenants and continued to resist Charlemagne throughout the conflict. He fled to the Danes when Charlemagne returned rapidly to Saxony in 776 but remained at large until the end of the wars.
Charlemagne ordered the beheading of 4,500 Saxons who had rebelled following the Battle of Süntel where Widukind won while Charles campaigned against the Sorbs. Alessandro Barbero states that claims denying this massacre or calling it a battle are not credible.
Charlemagne issued the Capitulatio de partibus Saxoniae in 785 which asserted that any Saxon concealing themselves unbaptized and wishing to remain pagan would be punished by death. This decree aimed to break Saxon resistance through capital punishment for observing heathen practices.
The last insurrection of the Engrian people occurred in 804 more than 30 years after Charlemagne's first campaign against them. This event marked the closing of the conflict as described by Einhard Charlemagne's biographer.