Complex sentence: Edward I mobilised another army for the Scottish campaign in 1307, which Prince Edward was due to join that summer, but the elderly King had been increasingly unwell and died on 7 July at Burgh by Sands.
Split into simpler sentences: Edward I mobilised another army for the Scottish campaign in 1307, which Prince Edward was due to join that summer. The elderly King had been increasingly unwell, however, and died on 7 July at Burgh by Sands.