He's a strange sans and in the wiki "which can Overpower Some Of The Multiverse Strongest Fighters, Such As Error 404"

And this is a attack and it's the only one that can do shit like this, adding that in the wiki 404 is a stranger 

"Bill was sucked into a strange dimension and encountered a white jacket entity. Bill was curious about that entity. The entity saw bill in its place and the entity attacked bill to banish bill from its place. Bill was hurt by the entity and immediately teleport away. Now, Bill still questions how did that entity hurt him." (wiki will be linked)

I'll stop talking about Bill!sans and now Error 404 the god of gods and the one who trained Error. First off let me say he was in a relationship already with Alphatale!Chara and as stated in the wiki

"404 and Alpha!Chara in the past had a very serious relationship finding each other one day in Hotland, they had some Ice cream and started getting closer from there. During one of the many genocides runs was about to finish off William when Chara stabbed Frisk in the back with his blades finishing off Frisk, Chara held William in his arms and reset the timeline and from there they hooked up and started dating. They spent over 4 years together and we're thinking of making each official until the siren went off. 404 has never forgotten Chara and refuses to date or grow romantically close with anyone for they only have the eyes for Chara." (expaining what their relationship is with each other)

See? He has his eyes on no one else then Alphatale!Chara.

https://alphatale.fandom.com/wiki/Error404!Sans

https://undertale-au-fanon.fandom.com/wiki/Sans/Bill!Sans#:~:text=Origin-,Bill!,brother%20is%20known%20as%20Will!