It was like being stabbed with dozens of pins in each nerve ending; Desmond's brain was under a great deal of stress from processing so much information. Hundreds of images run every second, showing Desmond an unfulfilled future.
A future where Desmond never came to Serefia, where Cecilia was alone. In that future, Cecilia managed to survive many tribulations on her own. However, some things did not change, such as Nana's death and the church's accusation of Cecilia.
A future where Cecilia was falsely accused and brought to trial. She was found guilty and sentenced to the guillotine for a public execution. Seeing Cecilia beheaded was even worse than Desmond's pain in his last battle in the plaza. Then Desmond saw it; the birth of a Celestial, and the vision ended.
When Desmond finally finished experiencing the vision, his body was drenched in sweat, and his breathing was a mess. A fistful of salt was probably less bitter than Desmond's expression at the moment.
