Sind only appears in the Known World fairly late on. You would think that everybody would know about the country, but it's hardly ever mentioned in the Gazetteers etc. Now you and I know that this is because the whole place hadn't been invented when the Gaz series was written, but you can't really say that to your players. So here's a (semi?) plausible reason why Sind only appeared recently.
The Sindhi Conspiracy
(or The Appearance of Sind on the World Stage)
by Colin D. Wilson
For many years there had been rumours of a collection of barely civilised tribes over the mountains to the West of Darokin and Glantri. The massive geological barriers of the Amsorak Mountain range, Lake Amsorak itself and the Atruaghin plateau prevented anyone checking such rumours, although Darokin caravans were seen departing through the one useable pass West of Akesoli.
They would return some months later with fantastic cargoes from the far West which, they claimed, they had transported across the desert and protected from the dreaded Sindhi tribes which preyed on all who entered their region. For the time involved and the cost of hiring guards and guides in the desert, the Merchant's Guild of Darokin would charge a healthy mark up on all the prices.
This continued for some years until, a few years ago (I'd put it about 995 AC, but it doesn't really matter), an adventurer visited the Ambassadors of most of the other countries in Darokin City. He claimed to have travelled secretly through the pass at Akesoli and discovered that the other side of the mountains was not so uncivilised after all. In fact, there was a highly developed nation, which called itself the United Mumlykets of Sind. He went on to explain how he had travelled around Sind, talked to it's people and been horrified to discover that the Darokin trade caravans were well known there.
What had been happening was that Merchants would arrive in Sind, spend a few months in luxury at Sayr Ulan, all the while buying items which were transported across the desert by *Sind* caravans. These items would then be loaded onto the Darokin packhorses, taken back to Akesoli and sold at an insane price!
Naturally, the Ambassadors were furious. Heated messages were passed between governments and only the Darokinian Chancellor's denial (under spell) that he knew anything about the practice was enough to stop a small war. This proved to the Merchant's guild that the memory adjustment spell they'd bought from Glantri was probably worth it!
Minrothad was particularly furious and attempted to destroy the Darokin land monopoly once and for all. Sadly, several of the more wily Darokinian merchants produced receipts for goods traded by Minroth merchants in the Sindhi ports of Jahore and Sambay. The Guilds became curiously silent after that.
Alphatia had always known of course but, since they could trade directly with Sind anyway, had thought it more amusing to leave Thyatis paying high prices.
Once things settled down, the 'new' country of Sind was added to the maps and brochures and trade documents. A new trading nation appeared practically overnight, and full diplomatic contact was quickly established. It tuned out that the Darokinians, Minrothaders and Alphatians had actually been pulling the same trick both ways, claiming to the Sindhis that there was nothing but waste beyond the mountains.
Sadly this new found economic and diplomatic prosperity only lasted a decade before Sind was hammered by the Master of Hule. Still, you can't win them all...