The masks that the boys find in the house, and the one worn by the ghost, are plague masks, notably worn by doctors dating back to the 14th century in Rome. These doctors treated people afflicted with the bubonic plague, also known as "The Black Death". The beak of the mask was hollow and was filled with dried flowers, herbs, and spices. This served two purposes, one of which was to mask the smell of dead bodies and other unpleasant odors one would encounter treating people afflicted with the plague. However, its main function was to protect the wearer from being infected by the disease. Back in the 14th century it was believed that diseases were transmitted via smell; this theory was called miasma. It was believed by doctors that the herbs would counter the "evil" smells of the plague and prevent them from becoming infected. However this was disproven by the germ theory of disease, but germ theory wasn't accepted by the medical community as fact until well into the 19th century.
Thanatology is the study of death. This particular field of study is named after Thanatos, one of the Greek gods and the personification of death, in Greek mythology Thanatos was analogous to the Grim Reaper. In terms of academics and science Thanatology specifically is the study of death as it pertains to human beings. It investigates the circumstances of a human's death, how the deceased loved one's deal and cope with the grief and the different attitudes, rituals and memorialization of the dead among different cultures and religions. It is an important topic of study for students of a number of different professions including: medical science, nursing, psychology, sociology, psychiatry, social work and veterinary science.
The scene on the staircase landing takes place in Hatley Castle, which is also used as the home of Oliver Queen in Arrow (2012) and Luthor mansion in Smallville (2001).
Alisen Richmond-Peck, who plays Penny Raider, also played Alice Cassity in Season 8 in Trial and Error (2013).
Sam thinks it is risky that Dean is willing to intentionally temporarily kill himself so that he can pierce the veil and get information from the dead, but Dean has already successfully done this not once, but twice before, in the Season 6 episode Appointment in Samarra (2010) and Season 11's Red Meat (2016).