It's a dim memory from many years ago, but the head of the convent either is an alien or has supernatural powers she uses to defeat the attackers. Does this ring a bell with anyone?
I think this is the 1982 story Souls by Joanna Russ.
The Abbess is the head of a convent and works to get rid of Norse invaders by seemingly magical means.
Near the end the Abbess is taken away by technological/magical means and tells the narrator she was here to study the people of the time.
I always thought she was a time traveler but an alien is possible - the narrator does not understand what they are seeing.
Maybe The Wind Eye by Robert Westall.
Plot: a family from the present day (1970s) are on holiday around Lindisfarne. The youngest child has a seriously injured hand, which was burned a few years back in an accident. Various strange things happen, then they are pulled back into the 7th Century, where they meet St Cuthbert aka Cuddy. Cuthbert can work miracles, because he is a Saint. He heals the child's hand and sinks an attacking Viking ship.
It was a monastry, not a convent, but similar