However, after his skull got broken and a part of his magic was taken by the now insanely violent Undyne, something within Sans changed. Sans instigated a new rule by Undyne, where humans who fall down are to be consumed for survival. After this, Sans was no longer the joyful skeleton he once was. According to the bunny, despite the rule he himself implemented, Sans has not eaten anything for 7 years, which then led to him losing his kindness, along with his sanity.
After many years of hunger Sans is seemingly bored of everything, even though the dire situation has damaged his psychological state severely, as he now seems to enjoy psychological humor and every bit of Aliza's screams. Whether it be Papyrus's puzzles or the hungry people of Snowdin or even himself torturing her, he has no sense of morale or seems to care about right or wrong; his jokes have changed from annoying yet funny to dark, insane and sadistic, reflecting his attitude.
He loves to decapitate humans for fun, usually using them for "head-dogs". Sans follows his own rule, which is to never eat a human, despite being the one who implemented it, and tries to prevent Undyne from finding out about Aliza's existence. He also shows regret and sadness over realizing that he almost gave into his hunger and tore out her arm to eat it, only for Papyrus to stop him via letters that told him not to eat anything that moves. This could somewhat prove that Undertale Sans is still somewhere inside the insane and starving skeleton. He is seen as the more psychotic of the bunch, even though he manages to speak and have normal conversations. Sans in fan works is mostly portrayed as a blood hungry and feral brute, that socializes very rarely, a far call from his Undertale counterpart though sometimes he's portrayed as a more psychotic version of Sans. Despite his psychotic tendencies, he sometimes shows signs of loneliness or hopelessness, mostly around his friends in Snowdin and sometimes even around his brother due to his introverted disposition.
Later it is revealed that although insane, Sans had been trying to earn the respect of the Snowdin Townsfolk by protecting them from Undyne and helping them find food.
In some fan works that include Horror!Sans now living on the surface, he doesn't act as insane as he did while trapped Underground and instead acts more aggressive and brute like.
Physical Strength - Horror!Sans is far more powerful than his original counterpart, with extreme physical strength, able to rip off the arm of a child in one swoop while also being able to cut through almost anything with his thigh bone.
Bones - While Horror!Sans still retains his bones, they now appear sharp with blood stains on them. Their unique abilities, however, are their extreme durability and capability to withstand almost anything without a scratch, as well as being able to instantly kill anyone from below, making them very dangerous when in close range combat.
Weaponry - Horror!Sans has several weapons along with him, his favorite being a giant, heavy thigh bone he pulls from the ground or a cleaver as these weapons tend to help Horror a lot.
Immortality - In many fan works, Horror is depicted having some form of immortality, claiming he can't be killed since he isn't alive, even though this isn't proven. Whether that is true or not, he is extremely difficult to kill, requiring complete obliteration to truly kill him. It is said, that, if any part of his body, no matter how small, survives, he is still alive, though this hasn't been proven or shown.
Semi-Immortality - As an undead, Horror Sans was implied by many to never age, and as a result, he stay young forever or at least never suffer the ravages of aging. Because of this, the user will always be at their physical prime. The user will also be immune to aging abilities, such as Age Acceleration or Reversal.
Gaster Blasters - While Horror!Sans has yet to use Gaster Blasters after having his eye removed, it is speculated that he may still have them.
Around one month after the CORE malfunctioned, and gave their food away to ones in need, while Sans convinced his friends that support and jokes would be enough to keep their spirits moving in the right direction. One day, some of the dog monsters left to go to the CORE for a meeting and asked Sans if he would want to come along before they left Grillby's. Sans turned down the offer and decided to eat instead. Suddenly, Grillby smashed his food, screamed from pain, as the clean air magic that created the fire of his very existence was now gone, and begged Sans for help. Scared, Sans ran out of the restaurant and decided to join the dogs to the CORE.
