The Night Creature at Storne Hope by Catherine Coulter

Thomas Oliver Maxwell Strickland (Max), the newly minted 6th Earl of Storne, wakes in his London apartment to find a young child hiding in his armoire. The child, who calls herself Crispin, is a runaway, but refuses to tell him any more of her story. Max, hopeful to help the child, invites her to stay at his new home, Storne Hope castle.
At Belhaven, Grayson Sherbrooke and Pip are expecting a visit from P.C., Miranda, and Barnaby, who now goes by Brady since being reunited with their family. Grayson learns that Max is with them, and will be taking up residence at Storne Hope castle, which is nearby at Piper’s Hill. Grayson tells the tragic history of Storne Hope and the ghost of Lady Hilda who still roams the rooms and halls of the residence.
When one night Crispin claims that she has been visited by Lady Hilda, Max enlists the help of Grayson to further understand the child’s secrets, and why Lady Hilda would take any interest. Grayson quickly learns the child hides a startling power, and that the evil she is running from is more terrifying and relentless than anyone could have imagined.