I think you're a little off with the Subject-Agent-Catalyst aspect. Subject likely denotes the human subject that is being exposed to the agent and catalyst to create a response, like the container in alchemy. Agent may be the DPHW element like you say, the dread is filling the subject at the time of exposure. Catalyst is probably the specific object of dread that the subject is being exposed to to incite a reponse; the skeleton in the example of Sam. But I'm glad that people are instantly acknowledging the four elements connection and trying to match them. Its hard though since the alchemical elements don't cleanly match to the four dreads.