We first met The Ones - later known as the Mortis Gods - in The Clone Wars as ancient beings thought to have been there when the Force first came into existence. The Father, who loves his children equally, represents the balance between the Light (The Daughter) and Dark (The Son).
Obi-Wan Kenobi, Anakin Skywalker, and Ahsoka Tano eventually encountered The Ones, learning The Father hoped Anakin might take his place due to the Jedi's conflicted nature. Instead, The Son accidentally killed The Daughter, and The Father took his own life to rob his Son of immortality (allowing Anakin to deliver a killing blow to the out-of-control God).
Ahsoka ended with Baylan Skoll standing atop a statue of The Father as he hunts down a source of power believed to be strong enough to bring peace to the Galaxy.
The prevailing theory is that the being calling to the former Jedi is The Mother, a.k.a. Abeloth.
An Expanded Universe/Legends character, she was a servant to The Ones who eventually took on a mother-like role in the family. Despite initially keeping the warring children in check, she fell into temptation and her own quest for immortality saw her draw from both the Font and Pool.
As a result, she became wildly powerful and transformed into a monstrous and near-unstoppable Dark Side being called Abeloth. The Ones, horrified by her betrayal and what The Mother had turned into, imprisoned her on an unnamed planet.
She remained safely imprisoned until they died and, with her cage weakened, Abeloth was ultimately freed by the son of Han Solo and Princess Leia, Jacen Solo (not to be confused with Jacen Syndulla). She wreaked havoc across the Galaxy as an even greater threat than Emperor Palpatine but was eventually stopped by Luke Skywalker.
Now we've brought you up to speed, what does any of this have to do with The Acolyte and, more specifically, The Master?
There's a popular theory online suggesting the mysterious villain is not a Sith, but a servant of the Mortis God, Abeloth. The basis for this appears to be the fact The Master's helmet has sharp teeth and a mouth which closely resembles how she's classically been portrayed in the EU.
We know the prequel series will introduce a coven of witches who use the Force but are neither Jedi nor Sith and they may also have ties to the villain trapped in a distant Galaxy.
In Ahsoka, we know Skoll isn't a Sith but is a Dark Side user and, aside from his red lightsaber, there's nothing that currently links The Master to the Sith lineage which eventually leads to Emperor Palpatine and Darth Vader. Instead, he may be one of Abeloth's first followers, tasked with weakening the Jedi so the Dark Side can rise again (heralding her return, in the process).
This would be a smart way to link The Acolyte to Dave Filoni's wider Star Wars plans and even links up to James Mangold's First Jedi movie which we're convinced will explore the Mortis Gods' origins.
Let us know your thoughts in the comments section below.