Complex sentence: Alexander had been in England in 1212, were he had been knighted by her father, and ha alleged that king John had promised him to give him Joan as a bride and Northumberland has her dowry.
Split into simpler sentences: Alexander had been in England in 1212, where he had been knighted by her father. It is alleged that King John had promised to give him Joan as a bride and Northumberland as her dowry.