Gunpowder fans were left trying to keep their dinner down thanks to the final, gruesome scene in the three-hour mini drama.
Kit Harington's character Robert Catesby, who happens to be Harington's real-life ancestor, was shot to death during an epic "last stand" against the monarchy before his severed head was placed on a spike.
The (very realistic) head made a squishing noise as it was stuck outside the battlements as a warning to any future conspirators.
Guy Fawkes had an even more grisly end: tortured on the rack, fingernails pulled out then hanged to death. Another plotter was disembowelled and had his heart torn out.
Speaking on the Jonathan Ross Show he explained that the trick was so brutal it left her "in tears" and saw her threaten to leave him if he ever pulled a similar stunt.
Video of the prank showed Leslie opening the fridge in her home, spotting the head inside before screaming and collapsing on the floor.
"It didn't go down well," a sheepish Harington told Ross. "She pretty much told me if I did it ever again that would be it - and I think that's marriage included."
While Harington's drama series has been undeniably gory, historians and critics have argued that it does history a disserve to "water down" factual events.
Writing for the Radio Times, historian Kate Williams said that, despite the violence in the show, the camera still spares its viewers from "the full horror of public disembowelling and torture".
She points out that Guy Fawkes was famously so weak after horrific torture that he could barely sign his confession.
Fellow conspirators were "hanged, taken down when still alive, disembowelled and their genitals cut off and burned in front of them".
Harington himself said he wanted to show a "very real, visceral and violent world" in the show.
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