The Creation of Rook.
Solas. Sigh. I'm proabably falling into this Regret Prison further than Rook, but frame-by-frame-ing the passage is just so thought provoking. And illuminating.
The Blood Magic. This is key here. I think, left to their own devices, Rook would fight Solas. Use the dagger to their advantage. Possibly get out of Solas' prison. It is still his, in fact. If he's telling the truth, a replacement is needed. He's made Rook suitable for the needs, as Solas sees them. He needs to break Rook to get the dagger.
Through the very powers Harding spoke of at the beginning (and we see in practice in DAI, to an extent), Solas is able to conjure the voices of the people Rook cares about the most, their found family. And Solas has them speak horrors that will tap into the regret he needs to overpower Rook.
He could turn Rook to stone right now, if he wanted Rook merely gone. He NEEDS Rook to trick the fade into holding them.
I also conjecture that Solas is making Rook hold on to the statues. Adding more and more weight as Rook becomes more exhausted. Rook fights it. They bury the dagger into the Veil, instictually. They turn to fight. But Solas is too powerful. Then he twist the proverbial dagger. Varric.
That stuns. That incapacitates long enough for Solas to manipulate Rook, literally this time, to loose their grip on the literal dagger and their work here is done.
Rook falls into the endless stream of their own regrets, as Solas cuts and runs.
And Rook's hands are empty, of the weight and of the way out.