I believe he is referring to Verin's final scene.

Verin’s Black Ajah identity was the most shocking revelation that Brandon learned while going through Jordan’s notes on the series.

This quote was taken from Brandon Sanderson’s Wheel of Time Answers From #TorChat.

I believe he is referring to Verin's final scene.

Verin’s Black Ajah identity was the most shocking revelation that Brandon learned while going through Jordan’s notes on the series.

This quote was taken from Brandon Sanderson’s Wheel of Time Answers From #TorChat.

EDIT: I believe my original answer is wrong. When I answered it, I was sure that scene happened on Towers of Midnight, but recently I re-read the final books and found out this scene is actually near the ending of The Gathering Storm, which came out in 2009, so Sanderson could have talked about it freely in that 2010 interview.

If I had to guess which scene he was really talking about, which has to be a scene from Towers of Midnight, I would guess it was

Aviendha experiencing the destruction of the Aiel in Rhuidean.

I believe he is referring to Verin's final scene.

Verin’s Black Ajah identity was the most shocking revelation that Brandon learned while going through Jordan’s notes on the series.

This quote was taken from Brandon Sanderson’s Wheel of Time Answers From #TorChat.

I believe he is referring to Verin's final scene.

Verin’s Black Ajah identity was the most shocking revelation that Brandon learned while going through Jordan’s notes on the series.

This quote was taken from Brandon Sanderson’s Wheel of Time Answers From #TorChat.