Ruins = a big old swamp
Snowdin = a dark desert filled with pits of lava and other dangerous things :^)
Waterfall = A cave filled with icy stalactites and mountain-like structures of snow.
Hotland = A bigger version of the CORE.
The possible intro to this AU would most likely be like this:
Once, there were two races. Humans and Monsters. One day, humans waged war on monsters, sealing them down into the depths of the underground with a magical barrier. The monsters scrambled for safety, as the underground was not a very safe place at the time. Eventually, monsters adapted to their new homes. Yet, they yearned for the surface. The king of the monsters executed any human that fell into the underground..However, he was defeated by a human, and the human was killed by guards before they could escape from the underground.
Mount Magmalite, 201X
Legends say that those who go to the mountain never come back..
insert Frisk falling
these are the roles:
Asgore = Asriel
Asgore, before his death, was what you know him as. A king trying to free the underground. He died from A blow from the Chara replacement, which would be Frara/Mirror (and no, Asgore did not brutally murder the mercy button).
Shyren = Toriel
Shyren, right after Asgore’s death, tried to lead a protest and make everyone believe that humans were safe, but the royal guard took over, and banished her (and some of her supporters) to the swamps, even with the support of the great celebrity of the underground. Shyren still sings, of course, and is dedicated to proving a point. She is still very shy like canon Undertale, but can be suddenly outgoing at times.
Mad Dummy = Napstablook
Mad Dummy is not very.. Mad. Instead, he is one of many monsters willing to take a human’s soul and free the underground. He possesses a suit of armor, instead of a dummy.
Pyrope = Sans
What an unusual switch! Pyrope is his same, heat-loving self. He is a typical prankster, and scares you when you get out of the ‘swamps’. He can use his own head as a flail in this AU. He cannot teleport. Instead, you have to walk back to Coffee’s [name to be changed]. (The coffee worker in Hotland replaces Grillby)
Muffet = Papyrus
Muffet is Muffet, but her fighting skills are Papyrus-tier. She is (In quotes) “dedicated” to freeing monsterkind, or, more importantly to Muffet, all the spiders. She sets up traps (mostly web-based) and even forces the player to jump over a (small) lava pit to reach her. Nevertheless, the player defeats her in battle, blah blah blah. Muffet is only a close friend of Pyrope, knowing him from previous pranks he’s done to her over the years. They are not siblings.
These are all the roles so far. There are more to come!