This question goes out to... Well, any stage of Gaster willing to answer, I suppose. I hope it's not too forward of me but... how did you get the holes in your hands? (Sorry if someone has already asked this as well...)
A shadow passes over Gaster’s features at the question and he bows his head, thinking.
“I suppose… that it won’t hurt to tell the story now. ” He seems to be talking more to himself. My research has long since been destroyed as it should have been, so there is no chance of anyone recreating my work…”
After a long while he looks up at the child and sighs.
“When I was younger… much younger, and we were all stuck in the Underground… after the Core was completed, my sights set on the barrier and an attempt to break it without having to collect more human souls.”
He looks into the distance, not really seeing, his voice growing soft, “Killing the human children that fell down… was breaking the King.
“So I did something…” He took a deep breath, “Well, it was unethical–unthinkable, actually. I… cut away the bone in my hands and used the pieces as raw material for the forms I hoped to create. I… my hope had been to create vessels for the royal children’s souls; to revive them somehow– and you have to understand, Asgore never knew of this. He still doesn’t know…” Gaster closed his eyes, “He must never know. I don’t know how he would react…”
He seems to lose his train of thought and shakes his head, “But, ah… as you can guess, my calculations didn’t anticipate the final outcome.
“I was blinded, I think… by my need to help the king, for a way to keep the Underground together, because monsters were quickly losing Hope.
“What happened, however, was that… I suppose it was putting too much of my own magic into the process, but… instead of just creating vessels, I created two new Souls as well as the bodies… and they became my two boys; Sans and Papyrus.”
Finally, Gaster is smiling, “And although my soul was irreversibly cracked in the process, I would never give them up for anything in this universe or the next.”