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”AC-498 Is a large gelatinous creaeure, measuring to around 25 cubic meters. However, this size may change periodically. It is unknown what AC-498 is comprised of, but his body matter is completely solid white. It has been reported that it has a sphere-shaped core within his body that glows a bright-orange, but there is no photographic evidence for it. AC-498 moves around by extending his body mass outwards and using it to pull himself in a direction, in a similar method that a amoeba moves using pseudopodia. His intentions are currently unclear, but it seems it’s current goal is advancing towards monsters.”

-Notes on AC-498 from the Royal Scientist, Dr. W.D. Gaster.


AC-498 is a large mass of unknown white slime. Contained within the slime are small air bubbles, which can be popped from the outside easily by applying pressure to the area the air bubbles are located at. While not clearly visible from the outside, if one either was able to delve far enough into the slime or cause AC-498 to stretch their slime to the point that it was able to be seen, then a bright-orange, glowing core would be revealed.


AC-498 has the inability to perceive or express emotions. In fact, the only desire that AC-498 has is the desire to pursue any nearby monsters, attempting to make contact. Due to this, AC-498 has an unending desire for new knowledge. This can lead to it being a destructive force against the underground if it were to obtain the powers and memories of an able fighter like Undyne or Asgore.


Research Notes

“During one of my experiments, I accidentally created some new creature. It’s made of a solid-white gelatinous substance. I have run tests on what said substance is, and all results have come blank. This must be some kind of new, undiscovered substance… I will have to do tests on this subject. I will give it the Code AC-498.”

“Today, I allowed subject AC-498 to roam freely in a confined, 400 cubic meter room. The way the subject move was in a similar motion to that of amoebas, extending it’s gelatinous body outwards to create pseudopodia, pulling themselves forward. For most of the time during the experiment, it simply wandered randomly. It did not react to any sounds created, any contact with the body or any odors wafting into the room. However, when a test subject was sent into the room, AC-498 immediately began moving towards the subject. No contact was made with AC-498 as of yet.”

“A test subject made contact with AC-498 today. The subject reported the contacted area to have a tingling sensation. Once retracted from AC-498’s contact, there was a thin layer of dust on the top of the contacted areas. I theorize that AC-498 starts to dissolve areas on a monster that makes contact with AC-498. However, I have no other substantial proof to back this theory… This will require more experiments.”

“AC-498 formed into an figure that is nearly identical to that of the previous test subject. It seemed to have the memories and intellect of the person it was imitating, being able to recall information only the test subject would know. The only difference between the two was the presence of numerous scars on his torso and arm. I would have to theorize that this is excessive body mass that was not needed to form the image, but was needed to be kept somewhere. Therefore, AC-498 created empty pockets to hold this excessive body mass. Upon surgically opening AC-498, 2 things were made clear. The first of which is that AC-498 has internal organs that function the same and are in every way identical to that of the test subject. The second is that, while copying the memories of the test subject, AC-498 does not have similar emotions to that of the test subject. Not only did it experience no sensations of pain during the surgical opening, but it has shown to show no emotions what so ever.”

“AC-498 has reverted to their original form. It has been shown to still contain the prior knowledge of the test subject. While it can occasionally morph into the shape of the test subject, it is most commonly that AC-498 will be seen in their original form.”

“AC-498 has escaped their containment area, and is currently roaming the facility, disguised as the test subject. I am currently having the entire facility evacuated. However, I have a horrible feeling that… It may have already left the facility. If this is true, then I must send out a warning to the entire underground. I have no idea the true potential this creature contains… It must be stopped before it is too late.”

End of Notes.


The battle with AC-498 is set in the CORE, and fluctuates every different run.

In pacifist, AC-498 cannot be damaged at all. Your attack is simply too low to reach through his body mass to the bright-orange core.

In the neutral run, you will do damage depending on how many monsters you killed. However, the more LV the better.

In the genocide run,will take 2 hits if you manage to dodge his attacks perfectly. If not, the battle will take longer.

When you start the battle, AC-498 will reach into the bullet box, expanding through it slowly until there is a small amount of space for the SOUL to remain. It uses this move his second attack as well. If you are hit by either one of the attacks, on his second turn, it will start molding himself into the form of the human. It will start by using a simple stick, a few slashes and jabs. However, the more you get hit, the farther up it goes, using the Toy Knife, Tough Gloves, Ballet Shoes, Notebook, Burnt Pan, and finally the Real Knife. As it goes, it start to lose more and more features of Frisk, the skin colors, eye color, etc. And it gains features similar to Chara, green and yellow-striped shirt, smile, red eyes, etc. 

Each of the different stages that AC-498 goes through has a different attack. 

The toy knife will summon multiple knives that rain down into the bullet box, dealing damage to the player and progressing AC-498 to the next stage, the Tough Gloves 

The Tough Gloves will cause two gloves to appear and slam into the bullet box separately, each of which causing the box and the player's SOUL to shake. 

The Ballet Shoes will start you in the middle of the two shoes, which will start spinning after a moment, and then eventually start moving, forcing the player to stay centered between the two with a very thin margin for error 

The Notebook will appear, not in the bullet box, but AS the bullet box, which will give the player a second to move out of the way, before it slams shut. This attack does marginally more damage than the previous attacks. 

The Burnt Pan will start to cook an elongated bullet box, which, occasionally, will be thrown into the air and quickly flipped. This attack will spawn grease bubbles which rise upwards, which the player has to dodge. Again, this attack does marginally more damaged than the previous attacks. 

The Real Knife will start by flashing half of the bullet box either orange or cyan, and the other half of the bullet box the same color. After giving the player a moment to move, the battle box will be cut into half (Note, if you're caught in this part of the attack, it IS an instant death, despite whether you're LV 1, or LV 20, somehow.) after this, the two halves will be shrunken to the point where the player can no longer move, and will be attacked simultaneously, one being orange, and one being cyan, corresponding to what the bullet box flashed beforehand. If the player is caught in the orange attack, it will do 20 damage on a neutral or genocide route, and 19 on a pacifist route.

To spare AC-498, all you have to do is dodge his attacks until he's nearly depleted his energy (roughly 14 turns). However, you can speed this process up by attacking, and it won't raise your LV (about 5-6 turns when constantly attacking)

Current Forms Accessible

Standard Form

The standard gelatinous form that AC takes under normal circumstances.

Science Researcher's Form

He takes the form of the Science researcher that was working under Gaster.


  • AC-498 was based off of the X parasite from the Metroid series.


If you want to give thanks, this is the place.


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