Shape of Nothing

Down deep, where no light penetrates, very little can survive... very little but this.

There exist creatures beyond mortal comprehension, in forms that defy sense and understanding. They move across dimensions, through minds, through awareness. They cannot be perceived, not by mortals. Not without the aid of magic.
In the darkest depths of the deepest seas, these beings linger. They are not the aboleths or the leviathans, nor the scylla or the brine dragons. They are invisible, perception-defying creatures that can only be defined by the absence of themselves - the shape of nothing, the absence of anything. To brush past them is to recognise the difference between nothing and something.
What little can be gleaned of them is that they exist, and that to be aware of their existence is to be in great danger. Detecting their presence allows them to also be made aware of the observer, and this seems to trigger a predatory instinct.
This is not an article documenting these creatures and their habits and habitats. This is a warning.

Basic Information

Anatomy

Hey, did you feel that?

— someone

Look at me by Hanhula (via Midjourney)

To correctly communicate about something, it must have a name - but as these creatures are impercievable masses of seeming nothingness, they cannot possess their own unique names. In lieu of better choices, they are referred to as the Shapes of Nothing. They are areas of space taking up areas from as small as 5cm² all the way up to kilometres-wide areas of utter emptiness, and have no recognisable or understandable anatomy.
The chief theory of their existence is that these are sentient areas of magic, perhaps tied tightly to negative energy, that have developed a violent awareness in the darkness and pressure of Istralar's abyssal depths.
They seem to oscillate in size somewhat at random - those few who have sacrificed their lives in studying these undersea horrors recorded observations of the absent areas suddenly increasing in size in response to external stimulus shortly before all communication ended.

Behaviour

Don't look at them, look through them. You cannot become aware that they're there. You can't even think about them. They're an infohazard.
If you're reading this from the ocean: get out of the water.

The shapes of nothingness seem to be relatively docile whilst unobserved, for there is no evidence of their involvement with life - aside from when they are very deeply involved with it, and from that, none survive.

However, due to their aberrant, mind-warping nature, this may be a deficiency in knowledge and understanding rather than a fact of their existence.
It is on awareness of them that they stir themselves into violence, shooting forth in the water at rapid speeds (that yet do not displace the water around them) to then eradicate their observer from existence. No trace is left of a victim's body.
Contact with the souls of those destroyed by nothingness in their afterlives has provided a worrying understanding that the areas of nothingness consume not only the body but also the mind, for the dead are left bereft of all memory. It is left to the psychopomps to understand the chronicles of their lives.
In extraplanar research circles, particularly across the Plane of Water, these travelling empties are warned against as symbols of influence beyond the stars - as leaking influence of the Void.
Mortal understandings do not document these influences out of fear of what attention it might bring - for as bad as these areas of nothingness are, there can always be worse.

Citizens of Krtajala by Hanhula (via Midjourney)

Warning! This article deals with horror, deep-sea topics, and imagery of both.

Scientific Name

Absens nihillum

Conservation Status

Unknown

Geographic Distribution

• Luari Ocean

Congratulations to you. You read too much, and now you know of the unknowable. Sweet dreams.

— someone

Deep Sea Understandings

The abyssopelagic and hadal zones of Istralar are not absent of sentient life, and indeed, many powerful spellcasters exist in those depths. The aboleths, for instance, are incredibly intelligent, if cruel, masters of those deeps.
Most of the knowledge these ancient, alien species hold is precious to them. They do not share it with other ocean-dwellers, not even in settlements like Krtajala, unless there is something to gain.
Warnings to not seek out the areas where naught existed have yet been one consistent message passed on from every deep-dwelling community since their earliest days making their presence known.
The message has ever been clear: these are threats that even milennia of deific interference cannot destroy.
These are a danger unlike any other.

Well, I'm not going in the ocean ever again now.

— adventurer

When it's quietest in the sea, y̵o̴ ̸u̶ ̵k̶n̴ ̵o̷w̴ ̴t̶ ̵h̸a̴t̷ ̷ ý̶̦o̵̰͕͗ ̷̧̛̑ű̵̩̀'̵̝̋r̴̰̲͂́e̴̹̦͑ ̸̘̪͆n̸̖̯̈́ȯ̴̧̃t̶̠͋͛ a̴̧͖̭̳̻̺̝̘̱̖͕̯̻̳͌̉̑́͝͝l̸̛̘̳͔̱̦̜̅̊̉̿ ̴̟͇̭̯̰̊̄̓̓̂̊̍͐̍̕͠ͅö̴̧̲̪̝́͑̓̀̂́͆̈́̌͘̕ͅn̵̛̰̰̰̜̺̥̠̥̗̥̹̻͗̀̇̆͜ ̷̢̧̛̺͚̩̘̜̲̜͖̀͗̃̀̔̓̕̚͝͝e̶̬̘̤̥̦͂̔͒͒̃͐