#he will also just leave you for a century wandering styx if you dont pay his fees

[oneirocide]

thinking of an undertale/underswap au where birds are the national symbol of the underground (a different bird for every region). they're symbols of freedom, prosperity, and strength--and, it is very important to mention, it began as propaganda. Gaster was experimenting on creating monsters capable of handling human-levels of determination without destabilising and figured using bird bones would allow a lab-made monster (sans and papyrus) to contain this without fusing human and monster genes and therefore creating (at best) a pile of semi-sentient goop.

these weapons would ideally be able to break the barrier and fight humans in place of monsters, with much higher success too. sans was designed with blue jay bones (for intelligence and aggression: blue jays eat mostly fruits/nuts/etc but do hunt small invertebrates and are known to be territorial and also decapitate other birds) while papyrus was created with the secretary bird (for raw strength and, again, aggression. just look at how that thing hunts).

the bros dont have wings but do grow feathers in various places and act like their bird counterparts. sans uses his hood to imitate the crest of a blue jay, only lowering it around papyrus. he is very protective of his brother, extremely intelligent and capable of mimicking the sounds of others eerily well. he also carries a hidden sadistic streak unexpected from what looks like a typical swap-variant. he also loves being high up and can often be found perched up in trees or on his brother.

papyrus is the quieter brother and also, can be a total jerk. he is a powerhouse, though dont underestimate his intelligence either--he's more than happy to take advantage of situations and knows his stuff. he is fiercely loyal and doesn't like large crowds. most assume he is not very active, but he likes to stake out his 'territory' often and is, in fact, terrifyingly fast when he wants to be. he is very good with children, too, and the first to adopt chara when she falls.

monsters even dress like birds. sans and papyrus are most noticeable, but undyne, alphys, mk, and a few others are noticeably dressed as well.

some of the swaps go like this

sans <--> papyrus (personality, blended roles)

undyne <--> alphys (personality)

asgore <--> toriel (personality, role)

muffet <--> grillby (role, blended personality)

gaster -> more aggressive/sadistic/insane, possibly gender swapped

chara (she/her) <--> frisk (they/them) (role swap)

mk (gender swap) etc

river person (nicknamed Charon/the Ferryman) -> was present during the human/monster war, got their name for their kill count and fondness of water. no one alive has ever seen their face