I have never see a Undertale + Fire Force AU so i decide to make one, the name was difficult to find because firetale and underforce was alraidy taken, so his name will be FireForcetale or FFtale for short. The basic of monster being stuck in the underground rest but is not because of magic, the monster are all infernal who are not dopelganger or people becoming infernal (which is strange) and can speak, born before the Great Cataclism , their body changing shape over the year (scale, fur, dog shape, rock shape,etc...) they even ave reproductive organ. They create their own society whith the object from the garbage which are from before and after the Cataclism, some basic Infernal and corpse of menber of the Fire Force fall sometime too. The Royal Gard job is like the original but they also do like the Fire Force because the fallen infernal would attack them too. If you have questions about the AU i answer them but for now here the design of Sans and Papyrus (i messe up the hat sorry).