High Infernal

The Infernal language is the language of devils, originating on the Infernal plane. The language has several different variations which are more separated by "level" than the ways that true dialects typically differ from one another, and while the fact that these varieties exist is acknowledged by most scholars, the route that Infernal has taken from its native plane to the Material one is poorly understood.
The culture of devils is sorted by a strict heirarchy and caste system, and this fact is reflected in the dialects of their language. The Infernal language that is inherently known and spoken by Tieflings and occasionally learned by other mortal races is not only one of the lower levels of the language, but it has also been altered in its trip across the planes. Both the nature and physiology of devils is markedly different from those on the Material plane, so it is an unavoidable fact that what is spoken by mortal races cannot be identical to its Infernal-plane equivalent.
The script used in Common Infernal is the same that was used in putting Abyssal to writing, and other uses on the Material plane, found in ruins and tomes, decorating weapons, even penning magical traps. It is generally understood to be "the" Infernal script, and the fact that there is one other Infernal variation found very occasionally on the Material plane is known by few.
A level above Common Infernal is High Infernal--though in the grand scheme of things it should be called Lesser Infernal, as there are levels above, but those are neither possible for mortals to learn nor to listen to without great distress. High Infernal uses two scripts that are entirely different from the symbols used in Common Infernal, starkly delineating Common Infernal as a lesser, derivative variation--despite the fact that it is still feared and uncomfortable for humans to learn.