Father Ernest was also known as Monsignor, is an old and flickering blind priest who lives isolated in St. Gall Boarding School, and is currently the head of the hospital that St. Gall has become. He is a very strict head master who is not afraid to severely punish students who he thinks deserve punishment. He seems to be very religious and wants all his students to act accordingly.
Father Ernest was also part of the Philosophy Club back in the days together with Jeremiah Devitt, Anthony Beechworth and Alexandre. In the end of chapter 2 Jeremiah discovers that Ernest also participated in the experiments related with breaching the veil (The Playwright) with Jeremiah and his friends back in the boarding school days. Furthermore Ernest never stopped with trying to discover what is beyond The Veil. Anthony described Ernest as being “one of the most renowned theologists” (Chapter 2), which would seem to imply that he was very sharp and composed. It’s implied that another priest (Father Eugene) became ill at the same time. It's not clearly known whether Eugene's illness was related to Ernest. It’s possible that Ernest stole the serum and used it on Eugene. When the priests abandoned St. Gall, only Ernest remained behind.
Sometime later, maybe in the 1880s, St. Gall was turned into a hospital. By that time, Ernest was a Monsignor. The nuns accepted Ernest as the head of the hospital, though they rarely saw him. Ernest instructed them to hire Frank Baldwin as a helping hand.
When Devitt investigated the hospital, he found records of six suspicious deaths that occurred in November and December 1891 and two deaths that were not necessarily suspicious (Chapter 2). Baldwin was eventually found guilty of murdering 14 hospital patients (Collector’s Edition). Since six of Baldwin’s murders occurred during the last two months of 1891, then most (or all) of the other eight murders might have occurred earlier in 1891. Devitt seemed to believe that the deaths had been occurring “lately” (Chapter 2). If the murders didn’t start until 1891, then maybe Ernest suddenly went completely mad in early 1891 (whereas before he had only been mentally ill). This is important because Anthony also went mad in early 1891.
Baldwin almost certainly injected his victims with serum. One of the patients spoke of “the darkness of his eye,” which matches Devitt’s description of the Simurg (Chapter 2). Kaufmann said that the nuns had found a hypodermic needle in Devitt’s coffin, along with traces of an unknown substance (Chapter 4). Ms. Parnell was also at St. Gall, and she apparently also entered the Veil (Collector’s Edition). Ernest said that he was trying to send his victims to the Simurg (Chapter 2).
It’s unclear how Ernest came to possess the serum. Anthony might have left it behind when the boarding school closed in 1876. But Devitt remembered Anthony as being very careful during his boarding school days (Chapter 2).
Ernest might have acquired the serum from Alexandre or another member of the Playwright. In St. Gall, there is a picture of Devitt’s graduating class, with one person scratched out. It doesn’t look like Alexandre is in the photo (Chapter 2). If Ernest was the one who scratched out Alexandre’s picture, there might have been some sort of connection between them. However, Alexandre had been in a trance since 1887 and Ernest probably did not use the serum until 1891. Also, the Playwright’s one rule is “see that no one knows” (Chapter 5). If they had given the serum to an unstable person like Ernest, they’d risk creating a public scandal (and Baldwin's actions did indeed become very public) (Chapter 4).
It’s possible that Ernest stole the serum or acquired it by accident. He might have gotten it from someone we haven’t seen yet. It's also possible that Devitt gave him the serum prior to suffering from amnesia.
Another possibility is that Ernest got the serum from Anthony. Some evidence suggests that they went mad at the same time. Also, Anthony explicitly told Devitt to go to St. Gall, where “they” would be waiting for him (Chapter 1). During Ernest’s autopsy, a pathologist found a piece of paper in his stomach. It was a list of names: Ernest Glynn, Jeremiah Devitt, Alexandre D., and Hugo Ashdown. Hugo’s name was crossed out (Collector’s Edition). This list may suggest a connection between Ernest and Anthony, since Ernest had the list and Anthony’s name wasn’t on it.
On the other hand, Ernest said, “I haven’t heard from any of you” (Chapter 2), which would seem to imply that he had no contact with anyone from Devitt's student group. It's possible that Ernest meant that he hadn't been in contact with the other three witnesses, but he *had* been in contact with Anthony, who was technically not a “witness.” In other words, maybe Ernest was trying to mislead Devitt without lying to him. A simpler explanation is that Ernest was unreliable. He might not have lied intentionally, but he was raving without thinking about what he was saying. In any event, whoever made the list knew to cross out Hugo’s name. Therefore, either Ernest knew about Hugo, or he was in contact with someone who knew about Hugo.
When Devitt approached Ernest’s room, the windows in the hallway leading to Ernest's room are broken under mysterious circumstances. It's possible that this is caused by someone's action through the Veil as a means to discourage Devitt from meeting the mentally unstable Ernest and being harmed by him or Baldwin.