Complex sentence: Margaret and the remaining Lancastrian nobles gathered their army in the north of England, and when York moved north to engage them, he and his second son Edmund were killed at the Battle of Wakefield in December 1460.
Split into simpler sentences: Margaret and the remaining Lancastrian nobles gathered their army in the north of England. When York moved north to engage them, he and his second son Edmund were killed at the Battle of Wakefield in December 1460.