Complex sentence: The original Bönpo Gonpa was later transformed into a Nyingma and a Kadampa temple at some stage but it wasn't until 1275 that it was founded as a Sakya monastery by Drogön Chögyal Phagpa on his return from China.
Split into simpler sentences: The original Bönpo Gonpa was transformed into a Nyingma and a Kadampa temple at some stage. In 1275 it was founded as a Sakya monastery by Drogön Chögyal Phagpa on his return from China.