Warning: This article contains spoilers for the seventh season of Game of Thrones

Catelyn Stark may have been killed off in season three, but diehard Game of Thrones fans are convinced she made an appearance in Spoils of War, the fourth episode of the seventh season of the show.

And, we have to say, we’re pretty convinced – especially as they have the footage to back up their theory.

Read more: Have you seen Game of Thrones without the CGI effects?

Taking to Reddit, one viewer has pleaded with internet users everywhere to go back and watch the phenomenal sparring scene between Arya Stark (Maisie Williams) and Brienne of Tarth (Gwendoline Christie).

As we all no doubt remember, Arya – impressed by Brienne’s prowess on the battlefield – approaches her and asks her to train with her. Brienne, mistaking the young Stark for a little girl, insists that it would be an unfair fight.

“Didn’t you vow to serve both my mother’s daughters?” asks Arya.

And here’s where things get interesting: when Arya says the word “mother”, a woman in a long green dress with long auburn wavy hair can be seen walking in the background. Her head is down and she’s moving fast – but she looks a lot like the late Lady of Winterfell.

Check it out (the moment you need occurs at 1.14):

“If it's unintentional, it's awesome,” says GrennsGal. “If not, more awesome. It means that Catelyn is checking in on Brienne and making sure she’s keeping her vows.”

A zombie version of Catelyn Stark isn’t that crazy a theory – particularly as we get exactly that in George RR Martin’s book series.

In the books, Catelyn’s naked body is thrown into the river after her throat is slit at the Red Wedding.

It is later found by the Brotherhood Without Banners and resurrected by Beric Dondarrion, who gives his life for hers through the last kiss of R'hllor.

However, she has been dead too long for the magic to work properly: mutilated and unable to speak, Catelyn is utterly consumed by vengeance and intent on killing each and every single person she believes to have betrayed her family.

She becomes known as Lady Stoneheart, mercilessly hunting down and hanging anyone associated with the Freys, Boltons, or Lannisters, even if they had nothing to do with the Red Wedding or if they are young boys, including Podrick Payne.

So could Lady Stoneheart finally be coming to the HBO show?

Well, probably not: just last year, Martin admitted that the character had been cut from the TV version and that he was too busy trying to finish the books, otherwise he would have been more involved with the series and found a way to include her.

Game of Thrones director Mike Mylod agreed, saying: “I've genuinely had no discussions about Lady Stoneheart, so I genuinely couldn't comment about whether that might or might not be in their minds for the future.”

Hmm. We guess it remains to be seen as to whether or not Michelle Fairley will be reprising her character, or an undead version of it, this season.

In the meantime, it’s nice to know that Catelyn’s spirit has found its way to Winterfell, eh?

Images: HBO