You attempt your first blow through the "mirror" with your knife, but instead fall through it into a yard, knocking the skull far away in the process, which disappears into nothingness.

A small warped fence blocks your path forward, though you could jump over it into the fields yonder.
You can also choose to walk left or right.
You look back, but all you see and feel is a solid stone wall with a drawing of a skull on it.