"They're so much like Gaster!"
".. you never met their mother."
For many years in the underground there was rumors/stories from around the time when peace was still between human and monster that Gaster had two assistants, the prodigy genius Alphys and the strongest(and smartest) warrior in the entire underground simply known as Justice, Gaster was in charge of basically running the underground's "government" but before that he was merely Asgore's most trusted scientist but his two students were almost as glorious as him in their feats, Alphys went on to become Gaster's successor in his first line of business and became the head scientist of the underground while Justice went on to become General of the royal army and founder/first grand master of the royal guard and gained a reputation for being the strongest(and most terrifying) in all the underground and for many centuries they were, after Asgore, the strongest in the underground until the day that Sans and Papyrus were born or rather Cosmo and Dusky were born.
The stories of those times always tell that Alphys may have been the successor of the great Gaster but that was simply because Justice refused! In fact Justice was by far the more extraordinary of the two, for what Alphys had compared to Justice in knowledge Justice was by far more creative and came up with many ingenious ideas that when successfully executed far surpassed Alphys accomplishments. Justice was the greatest inventor, engineer and architect the underground had ever seen but she was also the most mysterious person anyone could ever meet.
They say she randomly appeared in the underground not long before they were sealed away.. The underground was the place to be after all, the entrance was lined with a grand bazaar and all of it was quite ordinary in daylight until the night when the "sky" would light with millions of hanging lanterns, just past would be quaint villages and beyond would be acres and acres of unpopulated mythical "wilderness" beyond comprehension. It was any other normal day when the girl walked in the entrance, walking through the bazaar and stopping to help others on her journey, she stayed for a short while in a snowy village before leaving once more and she walked and walked and walked before coming to the very end of the underground and collapsing before the royal estate, the home of the royal army, the asylum of knowledge for the scientists and the royal palace.
Some say it was the two children of the royal family who found her out there while playing and others say it was King Asgore himself while privately speaking with Gaster but regardless she was brought in by the royal family and considered to be no different than the princess(female Chara) in circumstance though she didn't wish to be called princess but felt lady was perfectly suitable, and then it's said that the mysterious girl had all but abandoned royalty and instead sought to become student of Gaster and she did so successfully as well as her only acquaintance(at the time) in the entire underground outside of the royal estate, Alphys.
It's said when the prince and princess died she was at the front of the charge into the war, she wound up loosing an arm and nearly her life defending the underground and protecting the king, for the first five years sealed away she was not permitted to leave the royal estate even once, though many knew that was Gaster's decision, for personal reasons as well as needing to monitor her new arm.
After this she earned the title "Lady Justice" and so Justice became her name, she was the greatest knight that ever existed but to some who knew she was also very special in the eyes of Gaster, to him she was everything. And for many centuries after that he strived to discover how life could even be and eventually he would in the form of his, their, sons. Skeletons similar to Gaster but designed like a natural child, with characteristics of both a father and a mother, but he never did get to show her their greatest creation. For a tragedy struck, during a time of peace in the underground, where the great Gaster, Alphys and Justice reunited to continue their research together when suddenly as if the lock holding back hell snapped the barrier suddenly opened but not in the way it should've, the seal was like a portal now and the mass of energy flowing from it sent most all the way to the royal estate, it's said that the tragedy that occurred that day only stopped when a red and a green bolt of energy shot out from the general area of the asylum of knowledge. This was the day Cosmo and Dusky were born and the day Gaster and Justice vanished.
Toriel figured it was their doing, the colors fit them and all, so she went to find them immediately in Gaster's lab, but she never could've expected what she found.
"I figure Toriel will be the one on the receiving end of this note, and if so I deeply apologize for this burden I must place on you. You were...a wonderful mother Toriel...so I beg of you my Queen to be one once more. I know our departure is sudden but..oh Toriel you were her dearest confidant you of all people should understand how she pined to see the sun again, to return to her home, to live her human life, and I as I prepared for her a gift in hopes to make up for what she lost I found an opportunity to return her to everything she loved, and so I decided to not tell her about them. I planned to break the seal and send her through but if you are getting this letter instead of hearing this from me it means my hunch was correct and she made me go with her. She has seen my world for all this time, I wish to experience hers now. When you next see us Justice will be as she was but I will have changed, any sign I am a monster will be covered and simply by everything besides my appearance you will easily mistake me for being human as that is the role I now play. I chose to not tell her of their existence simply because I know her too well, if she knew she would've stayed. I love her too much to let her do that. To our sons, when I return do not worry, your mother will be close behind as she always has been for long ago she embraced the monster and she now lives forever, her one separation from her humanity. The elder son shall be named Sans and the younger Papyrus, Sans you shall hold to high expectations while you may give Papyrus an easy life, but make great things of our sons. Goodbye my friend."
Toriel almost immediately disregarded most of his requests, bringing home the two boys and declaring the new princes, Cosmo and Dusky Dreemurr, Cosmo would be raised by Toriel in the asylum of knowledge and would become the smartest monster in the underground but disavow the life of a scientist due to his distaste of his true father and would become an astrologist and astronomer instead, he always did have an obsession with the surface, while Dusky would be raised by Asgore in the barracks of the royal guard, trained since birth to essentially surpass his mother. Asgore and Toriel were always fighting after the death of their children and it's quite obvious it was sadly taken out on the skeleton boys. It wouldn't be until the savior Liz arrived and the Dreemurr family had three children once more that a group of allies would emerge and they would meet their illustrious leader Vengix who seems...oddly familiar.
A random MASSIVE lore dump of insane information that took me like three hours and I'm now on the floor finishing this...yeah I need to go to bed, enjoy. -Creepz