Not a lot of info about it, I'm afraid.
What little there is can be gleened from this thread at CandleKeep. There actually seems to be more information about it in Ed's head, but he hasn't found any way to use it yet.
I remember something about them using little statues as currency (FRA).
I remember there was a realm of lizardmen there and they fled into the tunnels of the Underdark.
I've used this place quite a lot, even altered the map, making the island a little bigger. The story was that the Northmen civilization flourished in the 11th century DR (like RW Normans) and they managed to take over Tharsult from the Calishites. The cultural mix caused that the island developed into something similar to medieval Sicily at its peak.
Not broken... missing. :embarrass
I meant to find the url and put-it in after I typed that, but forgot - It's there now.
"An island of trader folk right in the heart of the Shining Sea. Looks like chalk cliffs and little farms from the waves, but great harbors and tall cities are tucked away in it-- or rather, one big sprawling city that doesn't call itself a city. It's all warehouses and interlinking bridges and wealth, wealth, wealth. Those warehouses hold everything one can think of, trade rules and rivals meet here as equals. So long as you don't try open piracy or arson, the Tharsen are tolerant of you and everyone. It's a crossroads like Waterdeep. You've probably heard that they're crazed because they take statuettes as money. They take everything as money, and just use the statuettes as trade tokens to represent sums whose weight in coins would tear the bottom out of any purse. Intrigue, yes, and sharp trading -- but a place a merchant has to love. Sembia without the self-styled nobles and the nastiness."
("Ed Says: Geography")
-- George Krashos
"Tharsult isn't a heavily detailed locale in the Realms.
The 2e Forgotten Realms Adventures hardcover has a few brief mentions, as does the 3e Races of Faerun. There's the odd tidbit in Empires of the Shining Sea but these mainly relate Tharsult to places on the coast.
Additionally, Eric had this to say -
"Tharsult is not detailed in Serpent Kingdoms (at least I didn't write it up due to space considerations), although I have a few ideas regarding it. Maybe I'll put them together someday.
As for a tie between Tharsult and the scaly folk, discussions with Ed suggest it was once a lizardman kingdom, but the lizardmen have retreated into the Underdark via the hundreds of limestone tunnels that honeycomb the island. Their descendants now dwell in an isolated region of the Underdark beneath the island but may or may not be thriving.
--Eric"
We also know, thanks to the 2e and 3e tomes City of Splendors and City of Splendors: Waterdeep, that the Mhairaunn family -- who married into the Phylunds of Waterdeep -- are from Tharsult and breed watchspiders."
Hope you find this helpful.
DMD