SPOILER ALERT: This article contains major spoilers ...
Fans think they may have spotted a ghost in Game of Thrones' latest episode - and it could signal the return of a key character.
During this week's episode, The Spoils of War, Arya Stark finally returned to Winterfell and was able to show off her new sword-fighting skills to Brienne of Tarth.
During their sparring session, Arya reminded Brienne: "You swore to serve both my mother's daughters".
As Reddit user GrennsGal points out, when Arya delivers this line, the shadowy figure of a woman walks past in the background.
Fans think the shrouded figure could be the long-deceased Catelyn Stark.
"At the word MOTHER you can see a woman in a long green dress w/ long auburn wavy hair with her head down walking past the background gate, resembling Cat," wrote GrennsGal.
If the figure is indeed Catelyn Stark's ghost, it could mean the show is picking up a key storyline from George RR Martin's novels that until now has been left alone.
In the books, Catelyn is revived after her death and returns as a vengeance-seeking zombie by the name of Lady Stoneheart, hell-bent on wreaking havoc upon her enemies.
Stoneheart also plays a crucial role in Brienne and Podrick's storyline.
Game of Thrones showrunners David Benioff and DB Weiss had previously decided to not include Lady Stoneheart in the TV adaption - a choice that Martin told Time he argued against.
"That was probably the first major diversion of the show from the books and, you know, I argued against that, and David and Dan made that decision."
With just one and a half seasons left for Game of Thrones, it's hard to say whether Benioff and Weiss have decided to include Stoneheart at this stage.
If not, the figure could perhaps have been just a subtle nod to Martin's creation without actually writing her into the series. As Reddit user Microhive pointed out, it appears they made the effort to include the figure in two shots - once at 0:12 and again at 2:00 in the clip below.
Is it an easter egg, or a hint at a forthcoming return for the long-deceased Catelyn Stark? Only time will tell.
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*Game of Thrones airs Mondays at 1pm and again at 8.30pm on SoHo, and is available on NEON.