The Unrelenting Inertia is a Beast or a Kaiju that was sealed up in the Abyss. It has anomalies properties that stills the motion of all things even itself. It has a flame inside of its body that prevents it from freezing to death. It creates a deadly freezing gas that it floods in an area killing everything. 

The Unrelenting Inertia: The Beast of the Abyss

Name: Unrelenting Inertia
Type: Kaiju / Sea Monster
Location: The Abyssal Depths, at the very bottom of the world’s oceans.
Status: Sealed Away, but slowly awakening.

The Origin of the Unrelenting Inertia

Long before the rise of humanity and the age of gods, the sea was a vast, untamed wilderness filled with ancient creatures older than time itself. Among these primordial beasts, one stood out—a force of nature so terrible, so incomprehensible, that its very presence warped the fabric of reality around it.

It was said that the Unrelenting Inertia was not born of this world, but instead a cosmic anomaly that fell from the void between stars, crashing into the deep ocean with a power so great it shook the Earth to its very core. Some ancient texts spoke of its arrival, calling it "the stillness that never ceases." Its form was a swirling mass of incomprehensible matter, made of things that were once living but had long since ceased to exist—shadows of forgotten creatures, remains of extinct worlds. It resembled large fish or whale with glowing eyes that flickered like dying stars.

The Powers of Inertia

The Unrelenting Inertia was no ordinary sea monster. Its very nature was an anomaly, a paradox that defied the laws of physics and biology. It did not simply move through the ocean—it stilled it.

Everywhere it went, time and space would freeze, the motion of life would stop, and even the waters around it would stand still. Fish would become statues, frozen mid-swim. Waves would halt, their crash never reaching the shore. The very essence of life itself seemed to pause whenever the Unrelenting Inertia passed by, creating an unnerving silence that would drive sailors mad if they encountered it.

The creature's most deadly power, however, was its breath—a cold, suffocating gas that it exhaled from deep within its body. This gas was not simply cold, but paradoxically frozen in time. Anything it touched would become trapped in a state of suspended decay, frozen to death before it could even register what had happened. It was a slow, painful death, as if existence itself had been put on hold, and the world had forgotten to continue. This deadly breath could flood vast areas of ocean or land, killing everything in its wake, freezing both living and non-living things alike.

But the most terrifying aspect of the Unrelenting Inertia was that it could never die—because it was frozen in motion. No matter what harm came its way, it remained still, unyielding, and forever unchanging. Its very body seemed to reject decay, even as it decayed. A strange flame burned deep within it, a fire that should not exist in a creature made of ice, but it did. This flame kept the creature alive, allowing it to continue living in an unyielding, eternal stasis.

The Seal of the Abyss

For millennia, the Unrelenting Inertia's reign of stillness ravaged the seas and beyond. No ship could sail near its territory without falling victim to the stifling cold, and no creature could survive the frozen death it wrought. Ancient civilizations, the gods themselves, and beings from across the realms feared the monster's power.

Eventually, a coalition of powerful entities—divine, mortal, and otherworldly—decided that something must be done. A great ritual was performed to bind the Unrelenting Inertia into the deepest abyss of the ocean, a place where no light or living soul could ever reach. It was sealed away beneath layers of cursed stone and impenetrable waters, its power shackled by an eternal curse.

The ritual worked—for a time. But the Unrelenting Inertia was no ordinary beast, and it never stopped existing. It was not dead, but trapped, its flame still burning within, though its movements were halted. The stillness of the Abyss became its prison, the cold waters surrounding it like a tomb.

Awakening of the Stillness

Now, after countless millennia of imprisonment, the seal is beginning to weaken. The flame inside the Unrelenting Inertia still burns, its influence leaking into the depths of the oceans, causing unnatural storms and dead zones to appear across the seas. The once-quiet abyss has begun to shift.

Some fear that the Beast of the Abyss is stirring once again, its body slowly moving, its breath returning to the surface of the world. Ships that venture too far into the abyssal zones are now reporting strange phenomena—time freezing for mere moments, ships becoming stuck in place as if time itself had held its breath, and entire ecosystems being wiped out by strange, frosty gasses that linger in the waters for weeks.

Scientists are beginning to uncover ancient records and texts detailing the creature’s legacy. Some speak of a cataclysmic event when the Unrelenting Inertia will emerge fully from its prison, breaking the seals that bind it and unleashing its curse upon the Earth once more.

The question remains: will humanity and the world be prepared when the stillness returns, or will they fall to the unyielding freeze of the Unrelenting Inertia? And can anything stop a force that cannot move—because it has already stopped everything else?

The Future of Unrelenting Inertia

The beast still sleeps, but its influence spreads. Beneath the surface of the ocean, beyond human reach, Unrelenting Inertia lies in wait. And though it may not be able to move, its presence is already beginning to freeze the world around it.

As for the flame within the Unrelenting Inertia, it burns not only to keep it alive, but to remind the world that even stillness, once unleashed, is a force that cannot be stopped.

For the encounter, create a map preferably underwater but it could be aboveground. The map will include a spiral of pillars that is aligned in a circle, the pillars are scattered around. Each pilar gets progressively higher and smaller. The boss will move along a set path starting near the bottom and will slowly move up to the top sticking close to the pillars at all times. The main way to defeat this boss is to get within melee range making barbarians quite effective at doing damage to this boss. The curse of the abyss, shield burst, heat ray, and slam are methods of moving players away. The boss should take advantage by destroying pillars with slam and utilizing siege monster. The pillars have an AC of 14, have 4d10hp (20 hp), made of a mix of polished diorite and marble. The reason why you want to push players off pillars is to get them trapped in the progressively rising freezing sleeping gas. (Just make sure to raise the freezing sleeping gas up by 10ft each turn)

Lair Actions

On initiative count 2 (losing initiative ties), Unrelenting Inertia takes a lair action to cause one of the following effects; Unrelenting Inertia can’t use the same effect two rounds in a row:

• Collapse 2 pillars of choice with a snapping freeze from the gas. Increase the gravity creating a vacuum underwater or in air that pulls entities down to the ground doing double fall damage. Use the rules for Tremble and Fall as an example.
• Create an updraft of Unrelenting Inertia's sleeping gas, create 50ft tall, 10ft radius cylinder that rises from the point of origin of sleeping gas. Make a DEX saving throw of 18 to move out of the way of the updraft. Character can move up to 10ft to edge of the gas.

Regional Effects

The region containing a legendary Unrelenting Inertia's lair is Ley Line Interference, which creates one or more of the following effects:

•  For this encounter, if you have a random magic table that activates on spells that roll ones, then use it.
• All teleportation spells above 4th level will have a minimum casting times of 1 minute or higher.

The region containing a legendary Unrelenting Inertia's lair contains Curse of the Abyss, which creates one or more of the following effects:

Rules of Swimming with the Curse of the Abyss.

Having a swim speed is equivalent to having flight in this encounter. Flight doesn't work though. However there are limits to swimming in the abyss. A mysterious force pulls down on creatures that are not native to the farthest depths of the ocean. For each round of combat a player spends swimming without touching the ground, is however much the abyss pulls down on the player. The player will be pulled down 5ft on the second turn of swimming, 10ft on the third round, and 15ft on the 4th and so on. The curse of the abyss can have stronger versions of the pull mattering on the area where it lies. (Useful for a certain boss battle where players need to fight a leviathan but can’t swim up to it.)

Not to mention, swim speed is affected by these rules. It’s possible to become exhausted during prolonged swimming. You roll a Constitution saving throw (DC 10) for each hour spent swimming. Failing this saving throw will result in one level of exhaustion for the character. The rules place a hard limit on swimming for eight hours, specifically stating a character cannot swim for that long.

The players can technically take fall damage mattering on how long they fall for. The reason being that if the players fall/swim for long enough then the players will take fall damage.  If the players were to swim for 10 turns then they would be pushed down 50ft in the water. The players could swim up 30ft to counter balance this but at later turns, such as 20 turns, the player will be pushed 100ft down which is very dangerous. The feather fall spell does not prevent fall damage underwater. So you either better hope you have the health to spare, a swim speed or that you are a monk or rogue so you can easily reduce fall damage

If curse of the abyss takes place out of the water, make it so that flight each turn is subtracted by 10ft each turn. On turn 2 of flying subtract 10ft of flight, on turn 3 subtract 20ft, and so on. The curse gets reset back to 0ft of reduction when you touch stable ground.

If Unrelenting Inertia dies, these effects fade over the course of 3d6 days.