Secret Bastard Dungeon

From publisher blurb:

Duke Nigenard spent decades tirelessly in the pursuit of forbidden magicks to produce the ‘perfect heir.’ When he at last succeeded, she had him assassinated almost immediately by bricking him up in his secret laboratory. Centuries later, rumours persist that beneath the ruins of Lady Chrysocolla’s summer palace lies the old Duke’s dungeon, haunted by the Lady’s bastard siblings...