Isabella Ricci thinks they forced her from her home because she was a witch, and in 14th century Italy, that’s a death sentence.
However, when she realizes the boat is taking her to the notorious Povelgia Island, her fears about being found a witch are long gone.
Because Povelgia’s black hole is known as a dumping ground for the sick and dying, those afflicted with the Black Death ended up on Povelgia, and they never return.