The Big Bad needs one "in whom runs his blood" to complete his nefarious plan. The protagonist certainly is, as this is why "R" summons him/her there in the first place, but perhaps the enslaved spirit, and even "R" himself, are descendants too - the former possibly having lived in the mansion, and the latter's It Must Be Mine! obsession actually being a subconscious compulsion which drew him there. In any case, both failed either to fulfil the Big Bad's plan or to defeat him.
WMG / Mystery Of Mortlake Mansion