The word "Ipsilstanst" is simply Stofonian for "sea people", and is applied to these people because of their famed prowess at seafaring. No one knows what they called themselves.
The Ipsilstanst culture flourished so long ago that the earliest surviving records probably glimpse only its decline. Their heyday was probably about -1000 EYE.
Their base of operations as the coastal areas south of the Sea of Elos, from modern Mayola to Meelia. It is generally known that they were not politically unified. Nor did they attempt to conquer and occupy any territory that they visited.
Although they were known to be nearly unstoppable as raiders of coastal towns, they more often preferred trade to fighting. Their ships plied the entire known world, carrying rare and expensive goods from faraway ports. For centuries, they had a near-monopoly on the long-distance trade routes.
For such a well-traveled people, the Ipsilstanst were surprisingly insular. Sailors were not allowed to settle and remain in the countries they visited. Intermarriage and inter-breeding with locals was considered extremely offensive. As the trading ships were crewed entirely by males, who could be away from home for months and years at a time, this seems like it would be a difficult taboo to enforce. However, it seems to actually be the case that very few instances violations took place. Some writers claim that was because the men on the ships preferred each others company to that of the local women. Others say that the taboo also did not apply to male/female acts that could not possibly result in pregnancy.
Language and Writing
When communicating with other peoples, the Ipsilstanst preferred to learn the foreign language themselves and keep their own language hidden. For this reason, their language is still not deciphered.
When speaking in the languages they had learned, the Ipsilstant were famous for the use of Metaphorical language. For this reason, many suspect that the unknown language of the Ipsilstanst themselves was dominated by metaphor. This inspired the later "secret languages" of the Masters and Mistresses.
Fragments of Ipsilstant writing exist, but the language is so poorly understood that we do not even know what the words sounded like, much less what they mean.
Not being able to read the writing of the Ipsilstanst did not prevent other cultures from adapting their alphabet to their own uses. Both the Elosian Alphabet and the Old Mayol alphabet (the ancestor of the Stofonian Alphabet) contain letters that probably came from the Ipsilstanst. The borrowers did not know what sounds were originally represented by the letters they were borrowing, they simply assigned them to arbitrary sounds in their own language.
The preeminence of the Ipsilstanst as master sailors and traders was challeged by others peoples, who began to beat the Ipsilstanst using types of ships and other sailing knowledge initially learned from them.
Eventually the Ipsilstanst were unable to resist the movement of Mayols, Maaldobs, and Meelians into their home territory. Simultaneously, the Foels and Quoelpurn pushed up on them from the south. The isolationist habits of the Ipsilstanst caused their population and influence to steadily decline, as they found themselves more and more surrounded by foreigners.
It is not clear exactly when they became extinct, but they were certainly gone before the year 1 EYE.