Complex sentence: Blanche of Artois (1248--2 May 1302) was the queen regent of Navarre from 1274 to 1284, and later became Countess of Lancaster by marrying into the English royal family.
Split into simpler sentences: Blanche of Artois (1248--2 May 1302) was the queen consort of Navarre; after her husband Henry I of Navarre's death, she served as regent from 1274 to 1284. She later became Countess of Lancaster by marrying into the English royal family.