The Almighty (全知全能 lit. "All-Knowing All-Powerful") is the Schrift of Yhwach.

What it is

Like all Schrifts, The Almighty is an ability unique to the Quincy. They are each represented by a letter (or symbol). While both Yhwach and Ishida shares the same letter 'A', both of them have different Schrifts due to their different innate nature.

As the son of the Soul King, his letter awakens to be an ability that can see the future, one that the Soul King himself is capable of. While Yhwach has 3 pupils in his eye, the Soul King has 4, which might signify a greater extent of the ability.

When the Kanji of its name is read out directly, it is "All-Knowing All-Powerful". It can be translated to Omniscience and Omnipotence, which suggests two parts of its power.

The first ability "All-Knowing" allows him to forsee the distant future, and to "know" everything he sees. This might be how he countered Ichibei's Shikai of erasing his name. Yhwach can "know" his name by seeing into a possible future where he still owns his name.

The second ability "All-Powerful" is where all powers "take his side" when they cannot be used to kill or even harm him. This is how Ichibei's Bankai of giving him the name of an ant was peeled off.

It also allows him to "take and give all powers". When he gave his spiritual pressure that dwells in his sword to Ichigo, he could even command it to do what he intends.

Yhwach is also capable of self-fulfilling prophecies.

When Haschwalth borrowed the power, he admitted that his usage was no where near the extent or accuracy of Yhwach.

After Yhwach absorbed the Soul King, he was able to use the true power of The Almighty which is being able to alter the future he sees.

What it is Not

The Almighty is not not a reality warping ability. It is only an ability that allows the user to apply their existing powers to the future.

For example, Yhwach could revive his future self by using Auswahlen, another ability which he already owned. He gave his future self some power to revive.

While it nullifies powers, it does not nullify all physical phenomenon. Any direct attacks against his body will still affect him.

Strengths

Like Auswahlen, The Almighty also works on powers themselves, not Reishi nor Reiryoku.

Ichibei's powers are black, with no Reiatsu to be sensed from it. Yet, the Almighty was able to nullify it.

It is impossible to anticipate his attacks because the effect which takes place in the present is only initiated by Yhwach in the past.

According to Yhwach, he can see countless possible futures. This is basically 5-dimensional perception as not only he can see all possibilities over a range of time.

He can even see the futures when he wasn't physically present. That means he doesn't see the future through the eyes of his future self, but with the eyes of his current self.

Just by once being there, Yhwach could forsee Ichibei being resurrected there in the future, even though he wasn't there anymore when Ichibei was being resurrected.

By extension, Yhwach can revive himself even after he is dead, as it does not require his presence in the future, but just in the present.

The Almighty cannot be negated by any power that works on reversing the effects of time, such as Orihime's Soten Kishun, as The Almighty works "in the coming future".

With the power of the Soul King, Yhwach's power can apparently affect the future as far as one decade later.

Limitations

General

The effectiveness of the ability is limited by how powerful the user is. When it was lent to Haschwalth, he admitted that he could not predict accurately should the future changes.

The Almighty can be deactivated by the user's choice. While that gives Yhwach flexibility, he can sometimes be too arrogant and give his opponent chance to strike.

It is also possible for Yhwach to misinterpret what he sees. Yhwach thought that the time he saw Ichigo as a threat was merely a dream Haschwalth made him see.

Space Limitation

Yhwach can only see and change the future of where he is currently located at. While he can remember the future of the place he once was, Yhwach cannot intervene with the future he remembers, but only what he sees directly.

This limitation is clearly shown when Yhwach admitted that he didn't expect Aizen to show up in Soul Society.

This is because he only awakened the Almighty at the Soul King Palace, and has never been to Soul Society with his Almighty eyes prior to this moment.

This is also the reason why Aizen's Kanzen Saimin worked. If Yhwach were to be here prior, he would've noticed that the altered future he was looking at is different from the true future he remembers.

By extension, an omnidirectional shield may still be effective as Yhwach still has to be spatially located.

Time Limitation

The further in time, the more unclear it becomes. This is true for normal eyes in space, but more so for time as the number of possible futures also increases.

Possibility Limitation

While Yhwach can see countless possible futures, only one will eventually come true. It is possible for one to actually avoid the future altered by Yhwach. Yhwach can always alter multiple futures, but the more future he alter, the more power it will cost him.

The Almighty cannot foresee events caused by the Almighty itself before the change has been made. Meaning for at points in time A,B,C in sequence, Yhwach at point A cannot forsee Yhwach at point B altering an event at point C. However, Yhwach at point B can forsee what happens at point C.

Counters

Kanzen Saimin

Yhwach was made susceptible to Kanzen Saimin in his first meeting with Aizen. Therefore activating Kanzen Saimin before Yhwach even arrive at Soul Society prevents Yhwach from seeing the true future of that location. As that was the first time Yhwach has been to Soul Society after awakening the Almighty, he has no recollection of the true future of that location either. (Space limitation)

Book of the End

Tsukishima's Fullbring Book of the End can split the past of whoever it cuts by literally inserting himself into the target's history.

As a result, it created a past for Orihime's powers to reject until.

While Yhwach could forsee Ichigo showing up again with a fixed blade, he didn't know how it was fixed.

This is because Book of the End rewrites the past. Yhwach couldn't perceive such events via The Almighty before he left. And once he left the area, he could no longer see what happens and will happen there.

Anti-Thesis

Any ability that rewrites the past will also not be seen by The Almighty. For example, Uryu's Schrift "Anti-Thesis" is the ability to reverse two events that have already taken place between two designated points. It is the reason why Yhwach couldn't forsee his arrow of Still Silver.

Still Silver

While The Almighty can negate all powers, it cannot negate what's not a power. For example the Still Silver mixed with his blood cannot be called a power.

Another interpretation is that the arrow simply negated The Almighty first before The Almighty negates it.

Mimihagi's Stillness

Mimihagi is sometimes called acausal or undeterministic for not being detected by The Almighty, but I have a differing opinion on how it works.

Mimihagi is said to govern Stillness, the opposite power of Advancement. Mimihagi is undetected because it lacks entropy, the physical property that dictates the flow of time, making its past and future one and the same.

Coincidentally, the other power that negated The Almighty has to do with Still Silver, which is also an aspect of Stillness.