Complex sentence: As a result of this failed imprisonment, in 1420 the heirs of Joanna, Countess of Penthièvre surrendered Penthièvre to the Duke, thus ending one element of the Treaty of Guérande designed to favor Joanna and her heirs.
Split into simpler sentences: As a result of this failed imprisonment, in 1420 the heirs of Joanna, Countess of Penthièvre surrendered Penthièvre to the Duke. This ended the element of the Treaty of Guérande that was designed to favor Joanna and her heirs.