Ser Jared Frey claims the Robb Stark and the rest of the northmen turned into wolves and started attacking them so they killed them. However, he doesn't really expect them to believe his story and Davos doesn't.
The enormity of the lie made Davos gasp. "Is it your claim that Robb Stark killed Wendel Manderly?" he asked the Frey.
"And many more. Mine own son Tytos was amongst them, and my daughter's husband. When Stark changed into a wolf, his northmen did the same. The mark of the beast was on them all. Wargs birth other wargs with a bite, it is well-known. It was all my brothers and I could do to put them down before they slew us all."
The man was smirking as he told the tale. Davos wanted to peel his lips off with a knife. "Ser, may I have your name?"
"Ser Jared, of House Frey."
"Jared of House Frey, I name you liar."
A Dance with Dragons, Davos III
This is then discussed later on where it is made clear that they don't expect anyone to actually believe them but they believe they can get away with it.
The Lord of White Harbor leaned forward. "The Freys are no better. They speak of wargs and skinchangers and assert that it was Robb Stark who slew my Wendel. The arrogance of it! They do not expect the north to believe their lies, not truly, but they think we must pretend to believe or die. Roose Bolton lies about his part in the Red Wedding, and his bastard lies about the fall of Winterfell. And yet so long as they held Wylis I had no choice but to eat all this excrement and praise the taste."
A Dance with Dragons, Davos IV