Horror!boneliest (side canon) owned by ReiFumo, co-owned by... MEEEEEE

lore for now:
So we are located in the fight between sans and undyne in the horrortale timeline, undyne removes sans eye and puts it in the core to make it funtion, once sans recovered, he went to destroy the machine, but the moment he starts damaging it with his bones, the machine starts malfuntionating, affecting the eye and giving the same effect the core gave to gaster when he fell into his creation, making the eye an omnipresent living parasite until sans finally broke the machine, the eye returned into its position and sans left without noticing what he did.

Time passed and sans felt a presence was observing him, until one day the same presence called him into the core, he went there and took his eye back.

Once he put his eye where it belongs, he would start deforming slowly, papyrus realized this and told sans to cover up. Undyne doing her daily patrol realized sans was missing, instead, there was a monster covered in a cloak wandering around, after some pressure against papyrus he confessed that sans was hiding in the cloak.

Undyne decided to sneak attack sans, and after following him to waterfall, she confronted him and attacked, but was blocked by a bone covered with tendons and eyes, same tendons covered undyne up, undyne was confused about this, until sans revealed himself, showing his new form, the eye had a simbiotic relationship with sans giving him this new abilities, after that, sans escaped.