According to TV Guide, the two-hour episode will see Seattle Grace hospital filled with victims of a a multi-car pileup.
Meanwhile, one of the female doctors will have to fight a deadly fire in her home.
"Fans should brace themselves," creator Shonda Rhimes said. "That's the only way to put it - brace yourself.
"We're going to take you on a ride for the last two hours of the season and when the ride is over, none of our characters are going to be the same. It's very different in comparison to the other finales."
The season six finale of Grey's Anatomy will air on May 20 on ABC.
> Williams: 'Grey's finale is total shift'