Heroes face him on the Search Infernal's torture chamber for any sign of Statesman mission from Maria Jenkins. (Levels 45-50)
Defeating Infernal is required for the Dimensional Warder Badge.
On Praetorian Earth Infernal never had Numina's guidance to temper his drive to control more and more demons. When he first arrived on Praetorian Earth, Diabolique encouraged him to bind more and more demons into his armour until they corrupted his spirit. With the darkness came power and soon Infernal had become stronger than his guide. He is extremely power hungry and is happy to cross over to Primal Earth, where there may be more demons for him to use.