Crappy undertale au idea that came to me in a dream, and just a text post- Gonna name it here, "UnThinkTale"
Undertale but some of the monsters got killed and replaced by human-hunting impostors that copy their mannerisms to lure humans in to steal their souls and escape. By some i mean like Flowey (ig??) Toriel, Napstablook, Sans, Undyne, Mad/Mew Dummy and others. Flowey would probably stay "sapient" and behave almost exactly like the original except he's aware he's not just dust on a determined flower but something else. Asgore, Mettaton, Papyrus, Alphys, and such survived (Chara also 'survived' but that's because they're dead and a human.) The point is they don't ACT like they're going to murder you. Sans wouldn't offer to make YOU a hot dog. This isn't Horrortale. They retain the general personality of the characters they're imitating even to a detriment, like Toriel's kindness and Sans's patience. What they do is basically more like animals than evil clones, they just want 2 things. 1. To Leave the Underground, 2. To Survive. The intelligent personality is mostly just a shell borrowed from taking the original's place to make it easier, so for example, Sans making a new joke wouldn't be a sign of him being the original, but the quality of the joke might be. They'd continue attacking like nothing happened if you were about to SPARE them, and not even comment on it. hey might look a tad bit different to the original character in disturbing ways (like parts of clothing being magic'd into part of their body if they already weren't, Sans having a bit more teeth (lines) to his sprite) Chara being one of the few aware this is abnormal to the timeline, along with Frisk & Flowey's help and banking on using the 6 Souls to restore everything.