Pandy, a Hungarian-born clergyman, died in the medical center of the Leuven Central prison Sunday night, the Ministry of Justice said.
Pandy had been sent to Leuven for the slayings of six family members in the 1990s as well as the sexual assault and attempted murders of three adoptive daughters, the Belga news agency said in a translated report Tuesday.
The killings were carried out from 1989-1996 and revolved around a pattern of incest, Belga said.
Pandy's daughter, Agnes, helped Pandy dissolve the bodies in acid. The remains were never found.
Belga said Agnes Pandy, who later testified against her father, spent 21 years in prison before being paroled three years ago.