The Power of the Wouivre: Fiorenza by Emma Hine

Medieval Tuscany is a world of strict divisions – class, politics, gender, and religion – where everyone is bound to their place.
Guidaloste degli Orzotti, a young nobleman from 13th-century Siena, has always felt like an outsider. He avoids politics, has no cause to fight for, and loves a servant girl he can never have.
On his 18th birthday, he discovers his noble lineage and sets out to reclaim his family’s honour, only to find betrayal, deadly rivalries, and secrets that stain the Orzotti name.
Meanwhile, in 21st century Cornwall, English teen Thomas Ross only cares about surfing – until he discovers a power that lets him slip into Guidaloste’s mind, and a mysterious wouivre that binds them.

Can Thomas master the wouivre before ruthless enemies steal it and change both their timelines forever?