The big massacre. While standing there, looking at the brutal, heart-wrenching scenario, he went back into the wave of his thoughts.
He had a vision about this. He saw his mother and brother Robb in one of his visions. Though he can't remember what kind of a scene it was. In his dreams, mother and Robb were not even being pictured.
He has those kinds of dreams where he just feels about the person being present in it. He might hear some voices at times, but it's mostly the feeling presence of the people, without seeing them.
It was not rare having those kind of unexplainable dreams for Bran.

Sometimes, the dreams were just mysterious. People appearing in them which he never saw before. The unknown feeling connected to a person with an unknown presence. That's what they were sometimes.

And one of those had been the Three-Eyed-Raven. He actually accompanied him throughout his whole life. He just got to know about him much later.

And Meera. The first time he saw her, he totally knew she had been in one of his dreams before. She was the only exception though, where he didn't have the feeling of someone strange.

No. Although unknown, that feeling of her had been close to him. Like that of his family. He just could never figure out who she was, until he met her in real, seeing her in front of his very own eyes.

And she is left as his only support in this world without compassion. Without love. And without mercy.

As his eyes went over the picture, which displayed the brutal slaughter of so many innocent lives, a horrifying act caught his eye.

A fat, white mouse was roaming around desperately, nibbling at dead bodies.
It turned around his head in all directions, looking for something in particular.
In an instant she went into the hall where the disastrous incident took place.
He could neither identify Robb nor his mother between any of the passing bodies scattered uselessly on the floor. He heard a screaming little, screeching voice from the high table where Lord Frey himself is usually seated.

The white mouse was eating its younger version. It was tearing the little white mouse apart with its teeth, little by little, till the bigger white mouse itself was all covered in pitch red, the younger one left behind in little pieces massacred in the most horrifying way possible.

"The Rat Cook", he whispered, the picture getting blurred in front of his inner eyes.