> We're the infamous dog-burgulars, The Wet-Nose Bandits.
> (We were getting ready to sneak into that big house over the holidays...)
> When we were attacked from the shadows, by something MERCILESS!


So lets establish a timeline

>last holiday, so a decent amount of time before the game started
>The Dog gang led by Muttler fights the Roaring Knight outside the Holiday mansion
>This is when Muttler died
>a shard came off the knight's blade
>Asgore found it when cleaning up the grounds and this is what led him on his schizo search because it confirmed what he witnessed earlier with dark worlds was real

Further speculation: the knight is an amalgamate that needs to have new dust added to it to keep it alive. Shyra was the last sacrifice, the grave is new enough it wasn't there until after Susie's diner flashback.

Maybe the knight's "hand absorb" it was using on Toriel is used to absorb other monsters into it, while the undyne kidnapping was just to eliminate a witness, who coincidentally might also know about the shard the dogs found