The Realms of Adventure
The Twelve Princes of Shade claimed Sembia as a free protectorate following the disastrous civil war of the mid-1370s. In the years leading up to the Spellplague, that was the public face of things, at least. In reality, Sembia was utterly under the thumb of the Twelve Princes by the year 1380 DR, and by 1400 DR it was fully absorbed into Netheril.
Outsiders are not welcome in the floating cities of Shade and Sakkors, but the Sembian port cities of Saerloon, Selgaunt, and Yhaunn are busy shipping centers even on the diminished Sea of Fallen Stars. Netheril, gone from Toril for centuries, relied on Sembia to remake its economy and to provide sea ports and capital for trade, as well as soldiers and workers to protect and serve its needs. Though the Shadovar maintain a tight grip on Sembia, they have also grown dependent on it as their only link to the outside world. The canniest of the Sembian merchant princes have been able to capitalize on that fact, filling their own coffers with unprecedented riches.