The Ones
The Ones represent trio of beings known as Father, Son and Daughter.
In the mysterious realm of Mortis there exists a trio of beings able to wield the Force in ways no known mortals of the galaxy can. The Father maintained balance between his Daughter and his Son, who expressed affinity to the light and dark sides of the Force, respectively. The Father knew his days were numbered -- facing his impending demise, he needed to find another to keep the balance. His goal was the same described in an ancient Jedi prophesy -- the rise of a Chosen One who will bring balance to the Force.
Source: http://www.starwars.com/databank/father
The Son was embodiment of the dark side of the Force who dwelled on Mortis during the Clone Wars with his sister, the light-sider known as Daughter, and the powerful Force-user known as Father, who tried to keep them in balance. Son sought to escape Mortis and wreak havoc in the galaxy, trying repeatedly to lure Anakin Skywalker to the dark side by changing form, possessing Ahsoka Tano and finally showing Anakin a vision of terrible acts he would commit. His efforts ultimately failed, culminating in his death along with those of Daughter and Father.
Source: http://www.starwars.com/databank/son
The Daughter of Mortis is a Force-wielder who aligns herself with the light side of the Force. An enigmatic being of tall stature who can transform in to a griffin, she is selfless counterpart to her brother, the Son, who has allied himself with the dark side. Together they provide balance to the Force, a balance maintained and channeled through their Father.
Source: http://www.starwars.com/databank/daughter
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Ground realities of The Ones:-
# 1: Are The Ones really unstoppable?
1. The Father was in decline for some reason and would have eventually perished. This is why he was in search of another being to take his place and maintain balance between Son and Daughter.*
2. A weapon known as Dagger of Mortis could kill any member of The Ones with a single blow or swiftly.
Fearing that the Son may be unstoppable, the Daughter does the forbidden by taking Obi-Wan Kenobi to the Altar of Mortis, wherein is kept the Dagger of Mortis, a weapon capable of killing a Force-wielder. The Son steals the weapon, and attempts to use it against his Father to steal his power and end his rule, but the Daughter sacrifices herself, placing herself in front of the blade.
Source: http://www.starwars.com/tv-shows/clone-wars/altar-of-mortis
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The Jedi remain stranded on Mortis, and the Son, aligned with the dark side of the Force, renews his efforts to convert Anakin as the Jedi prepare for a decisive confrontation. Anakin is stunned by images of his dark future. The Son promises him the power to avert this destiny. The Father recognizes that the Son has broken the rules of time. He wipes Anakin's memory of these future visions, and steals the Mortis Dagger to end the conflict. The Father impales himself, thus preventing the Son from stealing his power. The Son, stunned by this, is run through by Anakin.
Source: http://www.starwars.com/tv-shows/clone-wars/ghosts-of-mortis
3. Son believed that he could defeat Father and Daughter with support from Anakin Skywalker.
Before the Jedi can leave Mortis, the Son takes Ahsoka captive in an attempt to entice Anakin into joining him to use their combined strength to overpower his Father and Sister. To this end, the Son casts Ahsoka under the spell of the dark side. Meanwhile, the Father attempts to stave off a disastrous showdown between his children and maintain the Force's increasingly precarious balance on the planet.
Source: http://www.starwars.com/tv-shows/clone-wars/altar-of-mortis
4. Anakin Skywalker could replace Father to maintain balance in Mortis.
A mysterious force draws Anakin, Obi-Wan and Ahsoka to a distant planet, and its inhabitants -- a family of exceptionally powerful Force-wielders -- in an attempt to determine whether Anakin is truly the Chosen One. The patriarch of this family, known only as the Father, has spent ages maintaining the balance between his Daughter, who is strong with the light side of the Force, and his Son, who aligns with the dark. The Father reveals his days are numbered, and he seeks Anakin to take his place as the fulcrum of this balance. A series of tests proves that Anakin is capable of controlling both offspring, as the Father does, but Skywalker refuses to take the Father's place.
Source: http://www.starwars.com/tv-shows/clone-wars/overlords
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*This revelation confirms that The Ones (Father, Son and Daughter) are not omnipotent or ever-lasting. They can perish from reasons other then a weapon designed to kill them.
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# 2: Did Father maintained balance of the Force on cosmic scale?
Let us re-examine some revelations from the official databank:-
The Father maintained balance between his Daughter and his Son, who expressed affinity to the light and dark sides of the Force, respectively.
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Meanwhile, the Father attempts to stave off a disastrous showdown between his children and maintain the Force's increasingly precarious balance on the planet.
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With all three Force-wielders destroyed, the imbalance in the Force disappears on Mortis.
Source: http://www.starwars.com/tv-shows/clone-wars/ghosts-of-mortis
These revelations imply that Father maintained balance in Mortis by keeping Son and Daughter under check. Nothing more.
Father did not maintain the balance of the Force on cosmic level. Some reasons:-
1. Darth Plagueis and Darth Sidious managed to influence the balance of the Force with joint effort by performing a ritual that lasted several months.
The question of whether he and Sidious had discovered something new or rediscovered something ancient was beside the point. All that mattered was that, almost a decade earlier, they had succeeded in willing the Force to shift and tip irrevocably to the dark side. Not a mere paradigm shift, but a tangible alteration that could be felt by anyone strong in the Force, and whether or not trained in the Sith or Jedi arts.
The shift had been the outcome of months of intense meditation, during which Plagueis and Sidious had sought to challenge the Force for sovereignty and suffuse the galaxy with the power of the dark side. Brazen and shameless, and at their own mortal peril, they had waged etheric war, anticipating that their own midi-chlorians, the Force’s proxy army, might marshal to boil their blood or stop the beating of their hearts. Risen out of themselves, discorporate and as a single entity, they had brought the power of their will to bear, asserting their sovereignty over the Force. No counterforce had risen against them. In what amounted to a state of rapture they knew that the Force had yielded, as if some deity had been tipped from its throne. On the fulcrum they had fashioned, the light side had dipped and the dark side had ascended.
Source: (Star Wars: Darth Plagueis)
2. Balance of the Force can shift depending upon various developments.
The sensation was completely alien. The Force was omnipresent. It radiated stronger in certain places and at certain times, and the balance of the dark side and the light constantly shifted. But it was always there in some way, shape, or form.
Source: (Star Wars: The Old Republic: Revan)
#3 Could Son and Daughter destroy the Universe, if they had left Mortis?
Father claimed that his children could destroy the Universe:
"And a prison. You cannot imagine what pain it is to have such love for your children and realize they could tear the very fabric of our universe."
First of all, this is an opinion of a character (not concrete evidence). Characters are known to deceive, manipulate, and lie to further their agenda. Father was most likely attempting to manipulate Anakin Skywalker to fulfill his agenda by replacing him for maintaining balance in the realm of Mortis with a scary claim. Father knew his days were numbered and he craved a replacement for himself to fulfill his agenda.
Son's known capabilities:-
Son have demonstrated the capability to shape-shift, possess a living being (telepathic talents), foresee future of living beings, unleash powerful bursts of lightning, formulate illusions and perform telekinetic actions. He also have demonstrated the capability to tank lightsaber stabs.
In single combat, Son dominated the trio of Anakin Skywalker, Obi-Wan Kenobi and Ahsoka Tano with his powers.
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Son undoubtedly have (very) impressive command of the Force, officially touted as embodiment of the dark side of the Force and even a god-like being. However, his supposed capability to destroy the Universe is based on a claim of his daddy so far, not concrete evidence.
If a being is capable of destroying the Universe, it should be able to break planets apart with mere thought or a gesture. It wouldn't even need to fight Jedi or living beings in general, everything would be beneath it. It is obvious from available information that Son is no where close to this level of capability.
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If Son had ventured outside Mortis, he would have caused trouble to the galaxy much like Abeloth. However, Abeloth's failure at subjugating Jedi and Sith is further proof of limitations of The Ones and beings like them.
Abeloth have history with The Ones, she forced The Ones to flee from a world where they all resided once but The Ones decided to imprison Abeloth within the MAW. For this purpose, The Ones (mainly Son and Daughter) joined hands with a species known as Killiks to construct a complex system to imprison Abeloth.