The slith tongue developed over the course of more than a millenium, its era-specific dialects typically classified into four parts: Archaic Slith, Classical Slith, Modern Slith, and Barbaric Slith.
Archaic Slith was spoken in the homeland prior to standardization, which was nearly one thousand years before the assassination of Hawthorne. Since it was not standardized, it was subject to many dialects and regional pronunciations and forms, although not as many as one might expect. Its unity led to the widespread belief that sliths in the homeland once spoke exactly the same language without dialectal differentiation, perhaps in the past so distant that it is now shrouded in mystery.
In any case, Archaic Slith was characterized by a handful of irregular or overlapping forms. It had an ablative case, which was already falling out of use late in the period, and it lacked a few of the verb forms that became available in the later language.
Classical Slith was the glorious form of the language spoken from the demise of Archaic Slith around nine hundred years before the assassination of Hawthorne. It was standardized not to any particular dialect of Archaic Slith (which is unusual), but to a mishmash dialect of all of them. The primary goal was to differentiate forms and smash down irregularities, so forms were put together that had never been spoken in conjunction before.
Classical Slith has been generally considered the most perfect form of the language. Although some (particularly poets) occasionally reached back to Archaic Slith for forms and constructions, the classical dialect was more often than not considered as elevated a form as was possible. The incredible number of forms of the verb, the simple declension system of the noun and adjective, the expressivity of the conjunctions, and the exactness of the language were held up as the standard for ages.
Still, over the course of the centuries that followed, the spoken language continued to evolve as the written language remained static. Eventually, after approximately five hundred years, the spoken language had moved so far beyond the standard written language that the language was standardized again to reflect more current usage. This new standard was called Modern Slith.
Modern Slith had fewer cases, fewer conjugations, and fewer sounds, but it balanced these deficiencies with an increase in prepositional usages and a wider vocabulary. It became the standard written language approximately four hundred years before the assassination of Hawthorne, although many with high literary ambitions still wrote in the classical tongue, because even during this period, Classical Slith was still understood, even if it was not commonly spoken.
This phase of the language was marked by more regularized constructions. Participles in oblique moods (the subjunctive, optative, and imperative) disappeared. Conditional clauses followed certain patterns, rather than being as free as in the classical tongue. Some long vowels were being lost, and a couple of cases dropped out of use.
Two hundred years before the assassination of Hawthorne, a very dramatic event in the history of slithkind occurred: the exile of Thsss. Many sliths were banished to Avernum, thus creating a linguistic divergence: the language of the Avernite Darklings began to evolve separately from the language of the homeland sliths. The Bahssikavans broke away from the Darklings on the very site of the banishment, which began their separate evolution as well.
Until their destruction shortly after the assassination of Hawthorne, the Bahssikavans continued to speak Modern Slith, and Thsss himself, a well-educated and literate slith, spoke that dialect as well. However, near the end of his life, and especially after his death, his followers began to speak a heavily colloquialized form of the language. In a few short decades, the language had undergone several more sound changes, strengthening stress and losing word-final distinctions, which wrought havoc on the declensional system.
Thus was born Barbaric Slith, a new dialect without declensions at all and with far fewer conjugations. Its vocabulary was greatly reduced as it became culturally unacceptable to talk about abstract or complicated topics beyond hunting and killing and war. Most Darklings retained an understanding of Modern Slith, but few spoke it anymore. In some particularly backward corners, knowledge of the older language was lost entirely.
Barbaric Slith was never standardized, which led (as with Archaic Slith) to a myriad of local variations, but the speech-patterns of the dynamic leader Sss-Thsss, descendant of Thsss, were extremely influential. This form dropped all distinctions between long vowels and short vowels, had a heavy enough stress that unstressed vowels underwent mutation, and (most colloquial of all!) introduced articles. Thsssish (a name given to the dialect by outsiders) became the most common dialect of Barbarian Slith.
Of particular note, also, is the Gnassish Dialect. This was created when, approximately forty years prior to the assassination of Hawthorne, a great number of sliths began defecting from the Darklings and making peace with the humans. They founded the city of Gnass. In an attempt to differentiate themselves from the Darklings, they began to change their cultural norms: it became acceptable again to speak abstractly, and they needed to expand the constructions and vocabulary in order to accomodate this. This led to a great deal of neologisms and strange new ways of expressing things, heavily influenced by human language.