Days without number - Robert Goddard

Michael Paleologus, retired archaeologist and supposed descendant of the last Emperors of Byzantium, lives alone in a remote and rambling house in Cornwall. His son, Nicholas, is summoned to resolve a dispute which threatens to set his brothers and sisters against their aged and irascible father. An overly generous offer has been made for the house, but
Michael refuses to sell.

Only after the stalemate is tragically broken do Nick and his siblings discover why their father was bound at all costs to reject the offer. Their desperate efforts to conceal the truth drag them into a deadly conflict with an unseen enemy, who seems as determined to force them into a confrontation with their family's past as he is to conceal his own identity.

Nick realizes that the only way to escape from the trap their persecutor has set for them is to hunt him down, wherever -- and whoever -- he may be. 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.