AU where Nightmare is an unfairly smart, pretentious prick who wants to take over the multiverse and then destroy it all. With some extensive conditioning and trial and error testing, he procures himself a henchman: a Sans that's Killer, Dust, and Asy, all merged together. They're energetic and loud sometimes, only ever thinking about killing, but then other times, they're too smart for their own good and they're more observant and analytical
Nightmare's plans get foiled time and time again by Error and Cross, of all people. Error is still a grumpy man child and he still kills people occasionally, but he's more laid back and is actually a genius. He's more likely to think before he acts, and he does his best to always plan ahead. Meanwhile, Cross is anxiety ridden and desperate to help Error figure things out, but Error never really gives him the chance, assuming he's incapable of doing much on his own
Eventually, Cross gets an arc where he snaps and goes mildly insane, and completely forgets what anxiety is. He does some bad things and suffers some guilt as a result, but very gradually learns to overcome his anxiety