The Infernal Machine is a collection of nanite-based semi-sentient space craft. They were originally created by the Draor to contain a madness inducing disease, but subsequently began to over-interpret their instructions and wander the galaxy.
Their discrete units -- effectively their ships -- are called modules. They fight with lasers and by releasing nanites that can consume and convert other vessels from within. They have defensive disruption fields that make them harder to harm, and can cut off access to hyperspace.
They are notoriously difficult to communicate with, and unreasonable when contacted. They view other ships as food. They posses a small number of "brain modules" that communicate, albeit with marked stubbornness. Other modules typically do not respond.
There is no empty space inside a machine module, and boarding them is not a well defined activity. They are known to be able to convert a ship infested with their nanites into one or more machine modules.
They are typically treated as more of a disease than a species, and the Earth Republic is usually in a fire-on-sight position towards them.