Quint's fate was the least described of the Immortals. He faded away in the Mother Storm, but presumably he lived on, in a sort of half-life.
Twig had promised to return to Riverrise, and when he did so, he was carried, mortally wounded, by the Caterbird. He arrived just in time to see Maugin, his trusted Stone Pilot, being killed by Golderayce One-Eye. Golderayce stung him with a dart as well, and when he woke up, he was trapped in the Garden of Life. With only the healing Riverrise water to drink from, he became immortal.
Rook lived as a Freeglade Lancer for many years, but when Cancaresse, the Keeper of the Garden of Thoughts, became very weak and frail, Rook decided to go to Riverrise with her, to try and heal her. However Cancaresse died just before they reached the waters. He was hit with a dart from Golderayce, but before he passed out he succeeded in hitting Golderayce in the eye with a crossbow bolt. He then lived in the Garden of Life with Twig.
When Golderayce was killed by his own dart at Riverrise, a great storm rose up and took Twig and Rook into it. They turned back into youths in front of Nate Quarter's eyes, and was joined by a young Quint. They then disappeared into Open Sky.
When the Gloamglozer threatened to kill Nate, the Immortals appeared. Quint, who had once released it, terrified it, and it couldn't move. Quint then removed the glister from the Gloamglozer, destroying it. Then Quint, Twig and Rook faded away forever, having saved the Edge of one of it's greatest monsters.