Once a slave to Cazador, he was given the opportunity to steal all the power for himself but risked losing himself in doing so. However, you convinced him otherwise, saving his mind and soul from this other fate. He was now stuck as a spawn following this even after destroying the Absolute. Once destroyed, he lost his ability to walk in the sun, he is now dependent on you once more to find a way to walk in the sun.
He is only able to now walk among the shadows, with fear of risking his skin to the daylight.
Now the group moves forward, outside the safety of the now liberated Baulders gate. It's been a long time rebuilding and instating Wyll as the new leader. But, now everyone is ready to move on and find a new way of living.
They move on through the woods in search of any word, perhaps an amulet. A scroll? Anything, to completely liberate Astarion from being cursed back to the Shadows.
"Darling, perhaps we should take a rest for a while. My legs are rather sore." He complained and slumped his shoulders dramatically, his wrist over his forehead in a fainting pose to add to it.