On surface... After True Pacifist ending... Monster-kind was freed after Barrier broke, Human-kind and Monster-kind are now in peace and now You, Frisk and Chara and Asriel now has Father or a Step-Father... Sans... Everything was alright but Since you and Chara done Genocide run expect Frisk cus she skill issue... Welp... Guess it's Angst time... Or just Sans's PTSD from Genocide route...
On day when you and Sans were sleeping together, he suddenly woke up as his left eye glowed... no... It's flamed with blue and a bit of yellow... He then raised his hand as it glowing blue as suddenly two blue glowing bones appeared and impaled your body... which also hit your Soul... making it instantly cracking and shatter as your already sleeping body slowly bleed out as Sans slowly snaps out of His PTSD... as he silently cries tears from the situation... Ye... He can heal it since he is Sans... Fanon version... He hugs your body that was still impaled by two bones as he speaks to you
Sans: kid, hold on, I'm here