When he arrived at the large and majestic castle that was at the far end of the Underground, he stopped to look around. There were no guards or armored monsters to greet him or prevent him from entering it. Just two massive doors in front.
Jay closed his eyes, breathed in and out, setting his mind on continuing forwards, then nodded before opening his eyes. He knocked on the door with the large handle, trying to see if anyone replied by opening it.

But nobody came.

[Did everyone run away and hide?] Jay wondered. He knocked again one more time. Still no reply.
Though the door opened a bit when Jay knocked again, as if it was unlocked on purpose. Jay opened the door and peeked inside of the castle.
The inside...was the corridor where he and Sans fought
The broken pieces of floor and roof have been removed while some damages were still visible. [I feel sorry for the reaction that the king might have had when he saw this castle room get thrashed.] Jay thought before entering. He walked forwards until he reached the end, there being a set of doors. Slowly and carefully, Jay opened it and stepped inside.
The room floor was covered with numerous amounts of yellow flowers, the windows from the sides and openings in the room letting in faint rays of light down. Birds were heard outside, the small chirping of their songs filling the air. In the middle of the room was a large golden throne with purple lining. And, a couple of feet in front of it....was King Asgore.

The king of monsters was knelt down near the flowers, watering all of them bit to bit while with a garden can and humming a song to himself.
Jay cleared his throat carefully and spoke. "Your Majesty?" Jay said.
"Oh? Is someone there? Just a moment, please. I have almost finished watering these flowers." King Asgore said. Jay nodded and let the king finish. Once done, the king of the monsters set the can down and turned around.
"Howdy! How can I—?" King Asgore began when he stopped and took a surprised step back, eyes widening.
"Yes, your majesty. It is I, and I am here in your presence." Jay said, bowing down respectfully and hoping it would calm the king down.
The king of the monsters simply said "Oh" and looked aside, seeming a bit in doubt.
"Heh. I...so badly want to say, "Would you like a cup of tea?" But...you know how it is now." King Asgore said after a long time of hesitation.
"I know, your majesty. I'd figure you must have come to know the truth. And I would like to apologize for lying about who I really was to you and your subjects. However...I chose something different from what I expected." Jay told the king reassuringly as he stood up again.
"Hm? And what may that be?" King Asgore asked.
"I have chosen to free all of the people down here and let them go to the surface. This is my choice alone. No one else forced me to choose otherwise nor have other plans to derail me from that choice. I hope...you could understand that." Jay said to the king.
There was a long moment of silence before Asgore spoke.
"I see. If so, then come with me. I'd like to show you something." King Asgore said before turning around and walking towards the back of the throne room. As Jay followed the king, he noticed at the right corner another throne, which was covered by a white sheet. [I wonder if this was the heir's throne seat.] Jay thought before he continued following King Asgore.
The king led him down a long and dark tunnel until they reached the end, which opened into an entirely new place.
This place led to another pathway, but it seemed way different. The path looked as if it pulsed with light and had the sound of wind as the light traveled on towards the end...which didn't seem to end exactly. It stretched on and on endlessly with no other path or exit in sight. Just a never-ending straight.
"This is the barrier. This is the thing that keeps all of monsterkind trapped underground." King Asgore began, his voice echoing down the endless passage. Then he turned around, and after that seven holes opened in between them in the floor. One vessel raised up from each hole, containing what appeared to be souls. Green, Yellow, Purple, Blue, and Light Blue. One of the vessels was empty with no soul in it. Immediately Jay realized what was about to happen; King Asgore was going to take his soul in order to break the barrier himself. Also...he felt as if the king misunderstood him. Jay was about to propose that he should stop his plan of starting the war and use the barrier neutralizer instead, not give his soul to him.
"Ready?" The king asked him.

. . .

"No." Jay said, words echoing solidly around them.

The king of monsters looked at him with disbelief in his eyes. "I heard what happened in the past, and...what you're about to do is not right. Starting a war with the humans won't change anything. And it certainly won't bring your son back. Yes, I also heard about him. Please, your majesty, hear me out. I can use the machine to break the barrier myself. There won't be a need to take another life just for the hopes and dreams of everyone. It'll just make things worse once people find out what happened to them and me. And...if I die...I won't keep the promise that I made with my friends here in the Underground. So please, your majesty. I beg you to not do this!" Jay said pleadingly, hoping his words could be heard.
There was a long pause when suddenly king Asgore took out a large red trident and slammed the blunt end on the ground firmly, making the ground shake.

"That will certainly not happen. I will not allow the hopes and dreams of every monster here to fade away because of a false promise you have made to them. Seven souls are the key to freedom, not a machine. Justice will be made once the war starts. I will not fail my people again since the day we were imprisoned down here. And I will not let you go away with the final soul needed!!!" King Asgore boomed, anger forming in his face suddenly.
Jay took a step back, scared because of how fast the king has changed. [He's not going to listen now. Asgore still believes that seven souls are the answer. Now...the only way to get through him....is by defeating him.] Jay thought, understanding the grief but resolve to do anything to free his people.
This was it. There was no other option but to fight King Asgore and stop him.
A strange light began to fill the room, twilight shining through the barrier. King Asgore was ready to attack with his trident as fireballs appeared all around him. And yet...Jay refused to give up right now. At this very moment, he was filled with something else.

Jay was filled with...

[DETERMINATION]