Walter's cries were heard from his crib by The Pack in Curse Of Strahd, but no baby was found. When the party of adventurers made their way into the sacrifice room, Walter was summoned to kill them in his new form, and almost did so. However, after swallowing Ghalwin Forrester, the paladin found the baby inside the mound and slashed open his throat.
This destroyed Walter, for the time being, but the undying Curse of Death House remained.