No, it's *not* called "The Excruciator" most of the time. There's a total of two times it's mentioned that I count, and it's called "the Excruciator" once, but that's in a context where any device might be called by the definite article. "The most gruesome of these devices was the Excruciator", just like "The most expensive of these dishes was the filet mignon". The other reference very much wants to emphasize the device's uniqueness but uses the indefinite article: "Can you imagine, human, what it must have been like to wear an Excruciator?"
It's not like there's much precedent in English for using the definite vs. indefinite article for simple emphasis anyway. "The" usually means there's one particular member of a group that's important -- not just some guy named Martin Luther King, but *the* Martin Luther King, or not just any house but *the* house (the one that's haunted) or something like that. That makes no sense here; the fact that Excruciators as a class are special is already reflected by the capital letter. Calling the whole class "The Excruciator" rather than "Excruciators" isn't a form of emphasis, it's just stilted.