Complex sentence: Gwenllian (1282--1337) was the only daughter of Llywelyn the Last, and to ensure the abolition of the line of Welsh princes, was abducted and imprisoned in a nunnery at Sempringham in Lincolnshire until she died.
Split into simpler sentences: Gwenllian (1282--1337) was the only daughter of Llywelyn the Last. After her father's death, she was held in a nunnery at Sempringham in Lincolnshire until she died.