Infernal language

Infernal was the language of the baatezu, a subtype of devils, and of the Nine Hells. Infernal was brought to Toril through contact with evil beings from other planes. It was described as harsh and alien in nature, since it developed among beings with thought patterns very unlike those of humanity. Clerics often took the time to learn it, and the Infernal alphabet was used to transcribe the Thayan dialect of the Mulhorandi language. Apart from being used for communication between the devils, in its written form it was also used to pen down infernal contracts.

Castes of Speech
The Infernal tongue was tied closely to the baatezu caste system, being divided into four parts. Though each part used the same basic foundation, each expressed different concepts and used different words.

The tongue of the least baatezu, such as the spined devils, was a simple, harsh tongue suited for shouting commands across a battlefield.

The tongue of the lesser baatezu was equal to any mortal tongue in complexity, used to communicate abstract concepts such as honor or hate. It was less gravelly than the tongue of the least baatezu, though not exactly musical. Most non-devils who learned Infernal learned this variation of it.

The stilted, formal, grating yet strangely melodic tongue of the greater baatezu was full of obscure patterns that seemed to meander misleadingly before snapping to an unexpected point. Speaking it properly required careful planning and the ability to respond intelligently to phrases based only on the most subtle of cues. It was said that two greater baatezu could hold entire conversations in this form of Infernal using only the beginnings of their sentences. It was nearly impossible for mortals to learn.

The fourth form of Infernal was the court language of Baator, used only by pit fiends and the archdevils. This form of the language was so utterly corrupt and evil that its malevolence could drag listeners into hateful despair just through hearing its patterns.