Battle of Tinchebrai
Henry had landed in Normandy with an invasion force earlier in the year. He was besieging the castle of Tinchebrai in southwest Normandy which was held by a supporter of Robert, when Robert decided to challenge him in battle, although Henry’s force was markedly superior in numbers. It was over barely an hour later and Robert captured. He was taken back to England and imprisoned at various places there, finally ending his days in Cardiff Castle, Wales.
Reference: The Oxford Companion to British History