Asgore Dreemurr is the king of the Underground and a boss monster in Undertale. He resides in "New Home", the monsters' current capital city. In order to destroy the magical barrier sealing the Underground away from the surface world, Asgore plans to collect seven souls from humans who fall into his realm through the cavern inside Mount Ebott. By the time the player character arrives in the Underground, Asgore already possesses six human souls.
Although most of the other monsters living in the Underground revere their kindly king, Asgore is wracked with regret over both the plight of his imprisoned subjects, as well as his own murderous actions to secure their freedom. His marriage to Queen Toriel also disintegrated after she became disgusted with Asgore's plan; she abdicated the throne and returned to the Ruins of the monsters' first capital city (simply called "Home") to live alone while trying to protect any humans who fell down from the surface.
The King's Decree
Asgore did not originally seek to destroy the barrier after the monsters were exiled to the Underground, desiring no further conflict with the powerful humans who had imprisoned them. One day a human child fell into the Underground and was found by Asgore's only son, Asriel, who quickly bonded with the lost human and persuaded his parents to adopt them as their second child. However, the child later became critically ill and died; their last wish was to see the "golden flowers" of their old village.
To fulfill their wish, Asriel secretly absorbed the human's soul after their death. With the power of a human soul, he was able to easily pass through the barrier and carried the corpse back to the center of child's village, where a bed of golden flowers had grown. Upon seeing the monster Asriel holding a human body, the villagers immediately attacked him, assuming that Asriel had murdered the child himself. Asriel did not fight back, eventually returning to the Underground and still carrying the corpse. Mortally wounded, Asriel collapsed and died after reaching New Home's castle.
The king and queen had lost both of their children in a single night. A vengeful King Asgore declared war on all of humanity and decreed that any new humans entering the Underground would be killed, and their souls harvested. He promises his subjects that once seven souls are collected and the barrier is broken, he will launch a full-scale assault on the surface world and reclaim what monsterkind once lost. However, Asgore does not appear to fully embrace his own agenda; he never attempts to pass through the barrier himself after obtaining a single human soul, which would have allowed him to collect additional souls much more quickly than by simply waiting for new humans to appear in the Underground.