Eroilin (Ay-roy-lin)

Once the language of worship, divinity and sanctity; now a language only learned in the shadows, and whispered in alleyways. Eroilin was, 500 years ago, the language by which mortals worshiped their deities. Prayers, religious hymns, ceremonies, tenets of faith and legends were all written or spoken in Eroilin. When the Gods, or whatever they were, were evicted from Duvrin, Eroilin disappeared with them. Within 50 years, it became a shameful language, never to be spoken in public.
When deities still ruled over Duvrin, Eroilin could be heard everywhere across all nations. Priests knew it best, some even using it as their main dialect. Most citizens also knew the language, though only few could speak it fluently, and mainly used it whenever dealing with divinity.
Even today, long after divinity's departure, Eroilin is not to be spoken. Given its association with religion, it has been lost to time. Very few are those who can read, let alone speak, Eroilin, and there are even fewer who would admit to it. Present day scholars do try to research and document what remains of the language but, even long after the Age of Demise, nations are reluctant to letting anyone look at the remains of religion, whether out of shame or paranoia.
In some parts of the world, words of Eroilin have remained, bleeding into the language. Tamynian swears are mostly derived from the dead language and, supposedly, a lot of Zilvran names take their meanings from Eroilin words, although historians are unable to confirm that rumor, as it would require a much better understanding of the dialect's legacy than the one they currently have

Dead, dead, dead

In present time, it is rare, even unthinkable, to hear or see Eroilin. The places that contain it are either ancient or secretive. Temple ruins sometimes have legends in Eroilin engraved on their walls, or old prayers hidden beneath layers of dust and dirt. Scholars might find an old hymn written within a book or a priest's journal, entirely written in Eroilin, recounting their daily routines and prayers. Other groups, heretics and cultists might aim to learn the language, thinking it could bring them closer to the old deities. Or, who knows, maybe the key to contacting those strange entities again is hidden within the knowledge of this millennia-old language.