Asriel Dreemurr is a central character in Undertale. He is a member of the collective of magical creatures called "monsters" that were long ago banished to the Underground by a magical barrier created by humans. As the son of King Asgore and Queen Toriel, Prince Asriel is also a "boss monster": a more powerful variety of bipedal monster well-suited for leadership. Raised by loving parents in the capital city of "New Home," he was known for his kind, carefree demeanor in spite of monsterkind's dire circumstances.
The Fallen Human
One day a human child appeared in the Ruins of the monster's original capital city of "Home," having fallen through the Underground's only known surface entrance at the peak of Mount Ebott. The human was found by Asriel and the two quickly became close friends. They were adopted by the royal family as their second child shortly afterward, leading to a renewed sense of hope for reconciliation with humanity throughout the Underground.
However, Asriel's human friend had a much darker side: they held an intense grudge against their own race. The child revealed their hatred of humanity only to Asriel and shared a plan with him to escape from the Underground. After deliberately ingesting poison, the human became ill and died, allowing Asriel to absorb their soul. When a monster absorbs a human soul, both souls share some amount of control over their host body. The monster also gains immense magical power in the process; enough to pass through the barrier themselves.
Carrying his friend's corpse back to their old village on the surface, the human residents attacked Asriel on sight, assuming that the monster had killed the child himself. Although his friend's consciousness inside Asriel wanted to use their new power to destroy the villagers, Asriel held back. The mortally-wounded Asriel returned peacefully to the Underground with the child's body, where he collapsed and turned to dust upon reaching the castle garden. Following Asriel's death, the human's corpse was interred inside the basement of New Home's castle. In a fit of grief and rage, King Asgore declared war against humanity soon afterward. He ordered the execution of any humans that appeared in the Underground; once seven human souls were gathered, they would be used to permanently destroy the barrier and attack the humans living on the surface.
Six more humans fell into the monsters' realm in the years that followed. Each one met their end at the hands of either Asgore's sentry guards or Asgore himself, who harvested the precious human souls in anticipation of destroying the barrier; however, no further humans would appear until the player character's arrival quite some time later. Although absorbing a single human soul would have allowed Asgore to cross the barrier himself and secure additional human souls more quickly, he was unwilling to do so. Instead he approached Dr. Alphys, the Royal Scientist, and asked her to research the true nature of souls by studying the six human souls that had already been acquired.
Alphys' experiments led to the isolation and extraction of a form of energy inherent to human souls that she refers to as "Determination": the power that allows human souls to persist after death. By injecting Determination into dying monsters, Alphys hoped to preserve and collect the monsters' more fragile souls after their bodies turned to dust. Because a monster cannot absorb another monster's soul, Alphys also sought out a non-monster candidate which could be used as a vessel for her test subjects' souls, eventually settling on a single golden flower from the surface that had bloomed in the king's garden after Asriel's death. She harvested several of its seeds for use as control cases during the experiment before injecting the flower with Determination as well.
Alphys grew desperate when her monster test subjects did not turn to dust as expected, leading to additional injections. However, each of the monsters instead made a full recovery. Her research into preserving monster souls after death was apparently a dead-end, but Alphys was comforted by the fact her test subjects could return to their lives. Unfortunately, the monsters' magical bodies were unable to sustain themselves individually with such concentrations of Determination. Their bodies melted and fused together, creating several monster chimeras called "Amalgamates". Panicked, Alphys sealed her lab to prevent the Amalgamates' discovery by anyone else. The golden flower that had been returned to Asgore's garden disappeared soon afterward, and a guilt-ridden Alphys eventually began to ignore the multitude of inquiries from the families of her test subjects, sending her into a severe state of depression.
Unbeknownst to Alphys, the flower she chose had grown from a seed that had stuck to Asriel's body during his visit to the surface. When Asriel died, some of his ashes -- Asriel's remaining magical essence -- scattered upon the seed, which grew into a flower after his death. With Alphys' injection of Determination, the plant became the vessel for Asriel's reincarnation; however, in this new form, Asriel's consciousness lacked a soul. Even after reuniting with his parents, the reborn Asriel was incapable of love. Without emotional connections, Asriel became despondent and eventually attempted suicide.
Even at the verge of death, Asriel still felt compelled to cling to life due to his Determination; suddenly, he found himself back in the castle garden at the very moment he had first "awoken" as a flower. Curious, Asriel began to experiment with his new ability to "SAVE". At first he used his power to travel through time in order to improve the lives of the Underground's inhabitants, but still he could feel no compassion for others. Growing bored with the predictability of his purgatorial existence, Asriel began to toy with the monsters in increasingly twisted ways, resetting reality from a previous save point when a particular scenario no longer interested him. Murder and chaos eventually became his primary sources of amusement; he also relished the power he gained from killing and embraced a creed of "kill or be killed" to justify his ruthlessness. At some point, Asriel completely shed his former identity and adopted the new moniker "Flowey" as he continued to terrorize the Underground for his own entertainment.
The Seventh Soul
Much time had passed since the last human's arrival in the Underground when a young child named Frisk finally appeared in the Ruins one day. Sensing the opportunity to steal a powerful soul, Flowey quickly intercepted Frisk and attempted to kill him. Flowey was driven off by the kindly Toriel, who had begun living in the Ruins shortly after her son's death in a vain attempt to protect any humans from Asgore. Like the previous arrivals, Frisk decided to return to the surface by passing through the magical barrier adjacent to New Home on the opposite side of the Underground. Heeding Toriel's advice, Frisk also strove to resolve conflict without killing and even made several monster allies along the way.
Frisk was confronted by Asgore upon his arrival at the castle. The monster king and Frisk had already clashed in an alternate timeline that had led to Asgore's death and Flowey's ascension to near-omnipotence. In this new reality, however, Frisk's undying mercy and compassion inspired Toriel and the Underground's other residents to rally against Asgore at his castle, changing fate and preventing the king's demise. Unfortunately, Flowey once again managed to acquire the six human souls, this time allowing him to also absorb the souls of every monster in the Underground. This enormous amount of soul energy restored Flowey to his original form as the boss monster Asriel. After greeting Frisk once again, he suddenly transformed into an adult version of himself clothed in a set of robes resembling those of his mother. As the self-styled "God of Hyperdeath," Asriel immediately initiated a cataclysmic event potent enough to destroy the world.
Now unfathomably powerful, Asriel focused the full force of his destructive magic at Frisk, the final obstacle to his complete control over time itself. No longer satisfied with merely snuffing out all of existence, Asriel declared that he would repeatedly reset the timeline so he could continue to torment Frisk for all eternity. Despite Asriel's fierce assault, the determined human's soul simply refused to die and withstood every attack. Annoyed with Frisk's resilience, Asriel changed himself into an even more powerful being: an abstracted configuration of shapes roughly resembling the Delta Rune. This bizzare state allowed Asriel to incapacitate Frisk's body with a thought and nullify his ability to SAVE. With no way to fight back, Frisk called out to his friends' souls still trapped inside Asriel. One by one, the monsters remembered their connections with Frisk and started to resonate against Asriel's dominance. Even after "SAVE"-ing all six of his friends, Frisk felt the presence of one final soul inside Asriel; a soul that he had never met before.
Frisk called out to the soul, and Asriel suddenly saw a vision of himself as a child, meeting his first human friend and being surrounded by his happy family. Asriel's resolve to fight began to waver as he experienced emotional empathy for the first time since his reincarnation as Flowey. Terrified, Asriel unleashed one final overwhelming attack in desperation, but the human continued to call out to him again and again while still clinging to life. Finally admitting his need to maintain any kind of contact with the player character out of fear and loneliness, Asriel reverted to his original child form and tearfully apologized for all of the senseless death and destruction he had caused. At last Asriel recognized how the human reminded him of his long-dead friend, after which he learned Frisk's real name.
With all of the Underground's souls inside him, Asriel now felt their love for each other and for Frisk, as well as their singular desire for freedom. Using the souls' unified power, Asriel destroyed the barrier and returned the souls he had absorbed to their rightful owners. However, without the souls' energy sustaining him, Asriel revealed that he would soon become a soulless flower again. After a heartfelt farewell, Frisk found himself back inside Asgore's castle and surrounded by his friends. None of the other monsters seemed to remember Asriel's resurrection, but somehow they all recognized that Frisk had saved the entire Underground. Together, Frisk and the monsters climbed to the surface to watch the sun rise over an expansive landscape, beginning a new day for monsters and humans alike.