[More speculation, but spoilers for those who haven’t watched season 3, episodes 13 – 17 of Star Wars The Clone Wars or season 6, episodes 11 – 13 of Star Wars Rebels or read the 3rd novel in the Aftermath trilogy by Chuck Wendig, “Empire’s End.”]
What I suspect is that the Son of Mortis survived his light saber wound from Anakin Skywalker, and that he was the cause of the malignant dark energy that Palpitine felt coming from the Unknown Regions in the novel Empire’s End. Darth Vader couldn’t detect it, but George Lucas has stated that while Palpitine had predicted to Yoda that Anakin would become twice as powerful as either of them, this didn’t happen because Vader’s injuries were just too severe. Lucas says that Vader only ever attained to 80% of Palpitine’s power.
(1) In any case, this dark power was so great that Palpitine had long range scout-droids sent out into the Unknown Region, and observatories built all through the Outer Rim, including one on Rey’s planet of Jakku. But it was something the Father of Mortis said that made me wonder. He said: “Deep within this planet (Mortis) exists a place where ALL that is dark is channeled.”
Nineteen to twenty years after Anakin visited Mortis is when Palpitine began to sense dark energy, then become completely obsessed with finding it. Then, as part of his Contingency Plan, in the event that the Empire should fall, he sent a group of Imperials towards the darkness. These Imperials later became the First Order. This why I suspect it must have been the Son manipulating this “Well of the Darkside” to bait them.
Now the three inhabitants of Mortis, the Father, the Son and the Daughter were servants of the Force and the Daughter described her family this way: “We are the ones who guard the power. We are the middle, the beginning and the end.”
The Force Ghost of Qui-Gon told Obi-Wan that Mortis was both a conduit and a magnet between the Living Force (biological mostly involving midi-chlorians) and the Cosmic or Unifying Force (electromagnetism, gravity, etcetera, folding of space-time).
Now, the Son on at least two occasions attempted to murder his father. (Echoes of Snoke egging Kylo Ren to kill Han Solo resound from this one fact).
In any case, the Father survived. The daughter used Force-healing to help him recuperate from the Son’s Force-lightning attack.
The second attempt was using the dagger that the Daughter directed Obi-Wan to fetch from the Altar of Mortis, saying that the dagger was the only way to stop her brother from leaving the planet.
That said, we know that the Son was able to get hold of the special dagger and with it go after his father with it, but the Daughter places herself between her father and brother and takes the blow from her brother’s strike instead.
Later the Daughter now dying gives what little is left of her life force to Ahsoka whom her brother murdered.
The Son is grief-stricken and shrieks up into the atmosphere, leaving the dagger behind, his sister being the only being he’d ever truly loved. (Echoes of Snoke egging Kylo on to murder Rey. Remember how Snoke told Rey that he’d kill her “with the cruelest stroke,” meaning by Kylo’s hand, as he had done to his sister. He was a real sadist.)
Later, the Father and Anakin plot to stop the Son, and at the final confrontation, the Father tells the Son that he wouldn’t be able to manifest his full power because the power of the Son and Daughter had to flow through him. It’s at this time that the Father plunges the dagger into his own chest. The Son is remorseful, and as his father is dying Anakin plunges his light saber into the Son.
The whole planet of Mortis begins to collapse due to the Father’s death, as he was the lynch pin, not his Son or Daughter.
Also, like with both Luke and Obi-Wan, at his death the Father’s body disappears and I’m supposing he becomes a Force Ghost. This is interesting because while the Daughter performed a selfless act, she does not become a Force Ghost, but we do see her later in another form, as Ahsoka Tano’s green owl companion, green being the colors of the daughter. Ahsoka calls the bird Morai.
We know this is the Daughter, or her as a Spirit Guide, because if you put on Closed Captioning, the hoot of the owl as it flies to Ahsoka’s arm is interpreted as “I am the Daughter.” Pretty sneaky of LucasFilmLtd/Disney.
But back to the son of Mortis.
(2) He’s the only one of the three that doesn’t get his mortal wound from the dagger.
I wonder if this was enough for him to have perhaps survived, remembering that the Daughter said it was the dagger, not a light saber that could bring him down? Too, the Son felt the dagger was the only weapon that would take down his Father.
(3) Then there’s what the Father said to Anakin: “We can take many forms. “The shapes we embody are merely a reflection of the life force around us.” So, this would mean that the Son could have used his Force ability to shape-change into the flying creature he used on at least three different occasions, making moving from one terrain on Mortis an easy task.
(4) Oh, and let me not forget that when the three Jedi were high up in the atmosphere of Mortis in their space ship, the Son was able jump aboard and kidnap Ahsoka.
(5) Well, we’ve seen both the Son and the Father just touch an ignited light saber and put it out like a candle flame with their fingers. The Son, when confronted by Obi-Wan, Anakin and Ahsoka, extinguished all three of their drawn light sabers in similar fashion. So, I have no confidence that a light saber plunged into his back could do any permanent damage, especially if he was expecting it. (Remember, we know that Kylo Ren was able to partially soften the blow of a bow caster into his side using the Force. Was this something Snoke taught him from experience? We’d seen in the movie Han Solo use the bow caster to kill two Storm Troopers in one shot, but Kylo survived what would have killed anyone else.)
But even if the three Jedi had suspected anything was amiss, there’d be no time for Anakin, Obi-Wan or Ahsoka to have done anything about it because after witnessing the planet fall apart for a minute or two, the three are whisked away to wake up on their space craft out in space with no memory of how they got aboard, but all their memories of everything else.
(6) Here’s the other thing I noticed. Although the inhabitants of Mortis are humanoid, they’re larger by a whole head. Anakin comes to the Daughter’s shoulder. This is like Snoke, a large-sized humanoid. I have yet to come across any other humanoid specie like those of the Father, the Son and the Daughter in the Star Wars universe except for Snoke.
(7) Then there’s the Son’s baldness. Like Snoke, the Son wore it well.
(8) Also, the inhabitants of Mortis dressed with a certain ornateness which is reminiscent of Snoke. Remember his golden Hugh Hefner robe, and his embroidered slippers.
(9) Too, if the Son did survive the chaos on Mortis, perhaps suffering severe injury in attempts to survive falling debris for a time, but then happening on an opportunity to escape, as he’d have to patiently wait for a transport to come close enough to Mortis, which is what the Father feared he’d do.
(10) Remember, too, the only reason the three Jedi ended up there was that the Father sent an S.O.S code known to the Jedi some 2000 years ago. Whatever apparatus the Father used, perhaps, the Son found and used to beckon to spacefarers not of Jedi origin.
(11) The corruption of the Son’s body could have had additional damage done to it after escaping Mortis. We’ve all seen what chronic use of the dark side of the Force can do to an organism. Just look at what happened to Palpitine in his duel with Mace Windu within seconds in the “Revenge of the Sith” movie.
Fortunately, if Snoke really was the Son of Mortis, being cut in half as opposed to being run through with a light saber, seems pretty final accept to, perhaps, someone in Darth Maul.
(This theory was originally posted on Reddit: https://www.reddit.com/r/starwarsspeculation/comments/9w5d2o/the_son_of_mortis_snoke_and_the_dagger_eleven/)