From Diablo Wiki
Infernal Machine is an end game quest feature introduced in the Patch 1.0.5. Players must hunt down and fight Key Wardens in order to get keys to create an Infernal Machine which can open a portal leading to a pair of bosses. There are 3 different portals that lead to 3 different pairs of bosses and as the portals are random it will likely have to be done more than 3 times. Meaning hunting down the Key Wardens for the 3 keys each time. The bosses can in turn drop body parts which can be used to craft a high powered Hellfire Ring.