The Dagger with Wings

Wealthy Essex landowner Augustus Aylmer confesses to Father Brown that fear of his impending death has caused him to dabble in the occult, enabled by John Strake, his adopted son. Aylmer had made Strake his heir because he feels his natural sons aren't needy, but ultimately realises his error and disinherits the necromancer. When two of his sons die under unusual circumstances, Brown suspects foul play.