If have a neutral or negative relationship with the Daemons, Toriel will ask you if you instead want to join her as her PRINCE at the end of her fight. If you say no, she kills herself before you can kill her. If you say yes, Dr. Muere will protest and Toriel will kill him for it. Then you get a few possible endings:
A more negative relationship means you become a ruler of fear and continue the tyranny of Toriel, Baby becoming the PRINCE of DARKNESS.
A more neutral relationship means the revolution eventually starts again and you and Toriel are overthrown.

If Baby leaves for the surface with all the stones, the underground is leaderless and a few different things can happen:
If you had a more negative reputation, the underground descends into chaos and anarchy, eventually destroying itself in war.
If you had a more neutral reputation, a general of the Great King will take over and the tyranny will continue, all daemons now void of hope.
If you had a more positive reputation, the revolution is successful and gradually, things improve for the kingdom.

If Baby stays underground, they take the throne. The following can then happen:
A more negative relationship means while there are some small improvements to the way of life, the underground is mostly unchanged. The Revolution became a Reign of Terror, with Baby at the front of it as a king of chaos.
A more neutral relationship means that Baby decides to establish a council of revolution leaders to handle the diplomatic situations. Life is gradually improving for the daemons, but there are still minor conflicts. The revolution's work is not done just yet. Baby strives to be a president of peace.
A more positive relationship means life for the daemon people shifts to become a utopia. Baby is immortal thanks to the power of the stones and becomes a benevolent emperor, reversing the damage Toriel had done. With Dr. Muere at their side, they become to the God of daemons, now content with their life.

If Baby breaks the barrier, the following can happen:
A more negative relationship means the daemons have grown to fear humanity completely. In the fear and anger that had built up for centuries, they immediately declare war. Humanity is wiped out almost completely, but so are the daemons, leaving Baby and Dr. Muere outcast and alone to travel together across the barren landscapes forever.
A more neutral relationship leads to the reveal that humanity went into hiding themselves after a large nuclear wars. Daemons and Baby can handle the toxic air and surroundings because of the way their body is made, so they end up taking the surface and starting a new life. Some are unhappy with how dreary it is though...

The true ending is breaking the barrier with a more positive relationship. In this ending, the daemons rise to the surface, though introduce themselves to the humans as the "monster" kingdom, wanting to distance from Toriel's reign. The humans are wary of a new sentient species out of nowhere, plus their magic and a mysterious "human", but are willing to make peace negotiations and slowly integrate. While Baby was intended to be the king because of their human appearance, they don't seem happy with that. Instead, the revolutionary leader before Baby takes over the kingdom. Now with the whole surface ahead of them, Baby and Dr. Muere want to explore the world together. They want to continue finding something out there that suits them right, get into all sorts of adventures. So with some supplies and money, they head out on the longest trip of their life, happily.