ART BY ARABELLEPRODUCTIONS!!
check her art here: https://www.tumblr.com/undertalerainbow

AU LORE:

Atlantis, kingdom of asgore, a place where monsters live peacefully, forced to destruction once the fear of humanity grew, a war that crushed the land underwater, drowning monsterkind forever, or so they thought.
While some monsters couldnt survive, the majority was able to adapt the circumstances, and decided to rebuild their lifes underwater, since if they go to surface, they will be targeted again.
The king asgore haves a plan in mind, with the power of the human souls, a profecy shall become a reality, a creature beyond our reality shall live, will carry on atlantis on its back, and will destroy everything on its path.

SOME QUESTIONS:

how are they alive?: monsters uses their magic to generate oxigen, some just evolved and some doesnt need to breath.

are they trapped?: no, they can exit atlantis whenever they want, they decided not to so they dont get targeted (people forgot they existed lol).

what about the souls?: instead of falling down, the 7 kids we know were taken by the waves of the ocean.

undyne must be very happy by living in the water, right?.... right?: most aquatic characters are in war with the oxigen-breathers because of territory lol.

ABOUT SANS:

the name " the fisherman" gived by the humans because he has been seen in the coasts fishing, humans doesnt know how it exactly looks, so hes a urban legend to them

hes grumpty, easily annoyed, enjoys fishing a lot, can use shortcuts, doesnt dodge since he literally fishes the attack button, (if you guys add him in some trio or some shit like that, you can make him block with his fish blaster that ill explain later) instead of changing gravity he makes you heavy enough to difficult your swimming, instead of kr he drowns you

ABOUT ANTHONY (canon name for blaster)

hes alive, sans fished it by accident and now they are budies, will protect sans at all cost, can eat whatever tf he can swallow, he can shot laser, he can get blind if he looks out of the water, and hes big af, if sans fished the attack button, anthony WILL eat it, you can get it back if you destroy him tho, dw.
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any other character will be added later on, any question, ask please.