He has recently been rediscovered as one of Swayne's prisoners within Mount Pillar. He had been kept imprisoned there for many years (possibly ever since his abduction), constantly forced to watch disturbing visions of the local Grisgol's tragic life, which rendered him catatonic. However, he recovered to an extent when he saw his son, although it took a little while for him to believe that he wasn't an illusion.
After Rygax saved The Party from Swaye, he sent William back to the Fadelands along with Zaheer and Daggerface, as they had not been affected by the Curse of Netharin and so were not plane-locked. Barring the possibility of their having encountered something Daggerface couldn't handle, William should therefore be safe in the Basilica of Saint Hitomi.