Days Without Number by Robert Goddard
Some secrets are best kept hidden... Nick Paleologus is summoned to resolve a dispute that threatens to tear his family apart. His father, Michael, is a retired archaeologist and supposed descendant of the last Emperors of Byzantium. Michael has received a hugely generous offer for the family home in Cornwall, but refuses to sell - and refuses to divulge why. But soon the stalemate between Nick's siblings and their father is tragically broken. And only then do they discover why their father was bound to protect the house at all costs. Their desperate efforts to conceal the truth drag them into a deadly conflict with an unseen and unknown enemy. Soon, Nick realises the only chance they have of escaping from their persecutor's trap is to hunt this ruthless adversary down. But the hunt involves excavating a terrible secret from their father's past. And once that secret is known, nothing will ever be the same again.