Gaster weakly reaches for the calming medications Alphys helped him make, his hands shaking as he opens the roughly constructed bottle. It didn't come from the surface. He takes a few of them quickly. He waits a few minutes for them to start having an effect.
"Everything we tried has failed," he thinks in despair. "Alphys tried so hard to help me, but it wasn't enough. We are no match for it. Not even close. All the ideas we had to free me haven't worked, and every avenue of research has turned into a dead end. I told Sans I was trying...but now there is nothing left to try..."
Except...that. He prides himself on being a man who can use his intelligence to solve almost any problem. But what do you do when there's no tricks left, no clever solutions? What if everything boils down and coalesces to a simple lack of moral courage?
Was doing something wrong to try to save his family justified? Even if he knows they'd rather die than have him save their lives by doing what he has? The answers seem to drift away and escape him.
He steadies himself, takes a long, deep breath, stands up and leaves his office. This part of the lab, where he lives now, is several levels down below the main level, and has been blocked off from other monsters, so that no one will see the state he's in. It's a relief not to have to worry about meeting anyone, or the entity harming them, but at the same time it's very lonely and isolating. The only one he ever sees anymore is Alphys. He keeps his head down and carefully avoids looking at any shiny surfaces on the hallways or elevators which might act as a mirror.
He takes an elevator to the deepest levels, his nervousness growing as the floors flash by. Every level the elevator descends makes him feel more trapped down here.
Can he really do this again? Hurt others again? It's all very well to say that he has the right to do whatever he has to, to protect his family, his own people. That logic has to have limits somewhere, though. Aren't some things wrong, no matter what?
He spends most of his waking hours looking for any kind of escape from the mental trap the entity has backed him into. Is it a lack of courage, or pragmatism that's keeping him trapped? Is it the lack of a clear answer, the worry that no matter what he does, several people could get badly hurt, including his family and friends? Lately he just wishes someone or something would either rescue him or end him, because he can't find a good answer. He fantasizes that the elevator will break and let him crash, ending it all quickly, the mental pressure building unbearably in his mind.
"Time to switch out for awhile, don't you think?", says the other, darker entity in his mind, feigning fake friendliness and concern. "Or do you want to keep feeling this way?"
He doesn't answer. "Don't pretend to care when you're the cause of all this," he thinks to himself.
"You liked this at first, not seeing what you were doing. Amnesia is preferable to guilt, don't you think?"
Gaster shudders, his skull flinching harshly to his right.
"Come on. You want to save the other monsters right? Just let me take over. I'll do all your dirty work for you. That's why you liked me in the beginning right? I was a tool you used, nothing more. A way to keep your conscience under control by letting you keep the truth out of sight and out of mind from your consciousness. You didn't realize I was more than capable of controlling you. It's your own fault, of course. The more LV you fed me, the stronger I became. Until it was too late."
Gaster reflexively starts reaching for his left hand with his right, only to have his hands forcibly controlled and pulled apart by the entity.
"Ah ah. Enough of that. I won't let you destroy yourself. Not completely anyway. It was a good try the other day, trying to throw yourself off the cliff before I could take control, and you almost caught me off guard. But I'm not losing such a convenient vessel so easily. I'm not done with you yet."
It was the wrong thing to say. The elevator arrives and Gaster gets out, and then stops in the hallway and thinks for a long moment. He catches his reflection in the shiny metal surface of the elevator doors, and this time, he doesn't turn away from it. He just looks at it, and accepts it. "I may look like complete crap, but I am more than a vessel. I am not a thing, I am a person. And because of that, I always have the ability to choose, and decide differently than I have in the past," he thinks to himself.
"Leave them alone," he finally whispers softly and firmly. "They are humans, but they are sentient beings. I cannot ignore that fact anymore. If you must torment them, you will do it on your own. Even if you threaten me with hurting my sons if I don't cooperate, there comes a point where a bully's threats must be ignored no matter the cost. How can I expect my sons to stand up to you if I don't? Even if you threaten to kill all of us, there are worse things than death, like betraying ourselves. I won't help you destroy my soul any more than I already have."
"Oh really? Then I don't need you anymore! You were useful for maintaining appearances, but now I might as well just take everything. I'll be you all the time. I suppose it doesn't matter to you if I start including your friends and family in the experiments as well? As long as Asgore keeps turning a blind eye to what goes on in the lab, they'll be easy prey for me."
The entity pushes him roughly back into a corner of his mind, taking over his actions and making him keep going to his original destination. Gaster fights and struggles to regain control, trying to turn around, get back in the elevator and return to the upper levels. But the entity grabs control of his soul and body and makes him keep walking where it wants him to go.
A heartless, cruel laugh echoes through his mind as he approaches the double doors leading to the most carefully guarded secret of the lab. His distress grows, and peaks, and then his mind blanks out and fades to black. He has the vague sense of time passing, as if he's sleeping, but he can't see or hear anything, and for all he knows, it could be hours, days...even weeks.
He knows nothing more for a very long time, until he hears Sans yelling at him. He's not sure exactly what Sans is saying, like he's hearing it underwater, but something in the feeling of it stirs him to try to swim upward through the murky depths of his mind. He evades the distracted entity's mental control long enough to break through to the surface, and suddenly all his senses come alive in a rush.
Gaster quickly evaluates the situation in the brief instant after he regains consciousness. He's suspended over the rift of that horrible device he and Alphys created, with Sans holding onto his soul with blue gravity magic nearby. It feels like a lightning bolt through Gaster's soul when he realizes Sans is visibly wounded in the area around his right eye socket. Despite that, Sans is clearly not giving up, resolutely holding onto Gaster's soul and not letting him go. It seems as though Sans is about to toss him into the rift. An impressive strategy, Gaster thinks, relieved that Sans appears to be winning. But Sans has a terribly conflicted look on his face. He looks very angry, but very sad and hurt as well. He's hesitating. He doesn't want to do it.
"PLEASE!" Gaster yells desperately in his mind, hoping Sans will understand. Even if it means eternal imprisonment in the void, he would rather that than let the entity roam free anymore, using him as a weapon against those he loves most. He can't make his voice work, because the entity still controls his body, but something in his eye lights or his soul seems to get through to Sans. He sees a flicker of understanding in his son's expression, and a deep well of hope rises up inside him as Sans's uplifted hand moves in the direction of the rift...