"mistakes were made."
– Sans the Seraphim
Seraphim Sans, also known as Omega!Sans, is a Sans that exists after killing Frisk and absorbing the human souls.
Seraphim Sans has a jawless Gaster blaster constantly floating above him, with a red eye to symbolize Frisk and Chara. He has a rainbow "Hoodie scarf," which has fur. Sans has 3 useless jaws that make up his neck, ending in a jawless skull. He also possesses 3 sets of wings. The pair at the bottom is those of the grim reaper, ending in purple tips and two eyes, Bravery and Perseverance. The middle pair is demon wings, blue and green in color, with two eyes again, Justice and Integrity. The topmost pair are bird wings, that are red and orange with two eyes, Kindness and Patience. Seraphim Sans wears dark shorts with light blue stripes. His teeth are very sharp and look like knives, and he has a long tail that ends with a giant bone scythe. His posture shows his ribs, in which is encased the soul representing Determination...Frisk's soul, Sans' own monster soul being in his abdomen. His arms are black, with a heart-shaped hole in them. His general appearance suggests that Sans has transcended concepts like "evil","good","monster" or "human", becoming a being close to the players own power.
Sans himself, personality-wise is still the same. But now he has all the eight fallen children inside him, each with their own personalities.
PATIENCE never wants to fight it out but is very social.
BRAVERY is always wanting to charge headlong into battle, with a fiery attitude.
JUSTICE is willing to FIGHT for whats right.
INTEGRITY is always being sassy.
PERSEVERANCE is anylitical and doesn't talk as much.
KINDNESS prefers MERCY and helps people feel better.
Frisk is regretful of all the resets and is always warm and friendly to Sans.
Chara is mostly uninterested and sarcastic. She goes crazy if given control of the whole body.
Sans retains all his previous powers but infused with the power of all the souls (since the souls fused with Sans, he has access to the soul's weapons and tools). When he uses the tools of the souls, the tools are colored, depending which soul used it.
He got the ability to reset or continue after being in Gaster's Dimension. [The void]
Frying pan - This is his first tool during the fight with Gaster. He uses this tool to Block, or Protect himself from Gaster's first attack. [The Prototype Gaster Blaster] When KINDNESS is with him, he can use this ability.
Gun - This is another Ability, he uses this ability to shoot during the Fight, the shape of the gun is like Empty Gun in Undertale. However, the difference between that is that it have bullets. When JUSTICE is with him, he can use this ability.
Ballet Shoes - This is another Ability, The ballet shoes function the same during Photoshop Flowey's fight, They go up and down when INTEGRITY is with him, he can use this ability.
Tough Glove - This is another ability, the Gloves function hitting the Opponent several times, depending how many times you punch, the more damage that your Opponent's HP Lower. When BRAVERY is with him, he can use this ability.
Notebook - The “attack” is more similar to what the item does in the game, which is that it increases your evasion while having really poor attack stats. 
Real knife - When Sans is tired, and Chara is in full control, They can use this Ability. The damage is greater than the other abilities, the Real knife is big, shaped like a Scythe, it also has a flower on it, Resembling That/how Chara ate the buttercups or Resembling Asriel. Only Chara can have the ability.
Toy Knife - This attack utilizes little plastic knives that only show up at the most opportune moment in the fight. 
Jawless Gaster Blaster - This is his last thing, its not an ability since neither Sans nor Chara can control it. The Gaster blaster will block Seraphim Sans' way, making the Opponent hit it instead, Frisk controls it.
After a genocide route, Sans becomes fearful that Frisk would never come back after a genocide route. In a state of panic Sans kills Frisk and takes their SOUL at the beginning of the timeline, and when Asgore refuses to fuse with the other six to free them, he does so himself, believing that this is the best for all monster-kind. He then breaks the barrier.
Meanwhile, Papyrus discovers Frisk's corpse. Not knowing how human death works, he takes it to Undyne, who goes along with it as to not upset him. They both go to ask Alphys about where the SOUL and Sans went, and they rush over to the castle. There Undyne tries to assault a hulking beast, who turns out to be Sans in his new form.
Toriel then arrives. Furious at Frisk's death and believing it's Asgore's fault, she launches an enraged and heartbroken assault. She recognizes Papyrus's name from her conversations with Sans and gets stopped from her useless attacks by a certain fish suplexing her because she can.