Briefly skimming, I found him in the second Knights book "The Swords of Dragonfire"
Near the end of the book, Eirhaun sends some low level Zhent wizards after the knights. Old Ghost and Horaundoon possess them into spell-fighting each other. Magic goes wild. Eventually they all attack Eirhaun - their magic explodes and his magical shieldings go wild and he is horribly disfigured. I assume this is how he became "the maimed wizard"
Then Sarhthor sends them all to the abyss and Eirhaun drags him along to. In Sarhthor's later report to Manshoon and the Shadowsil, he and Eirhaun make it back from the abyss. No information about whether Old Ghost and Horaundoon do as well, however.