The Imaskar were bigbad spellcasters. One day, they decided gods weren't worthy of worship and slain their whole pantheons. Then they decided to get their work done by slaves from other planes, introducing notably the mulhorandi and untherites. They sealed off communication between the plane they took their slaves and Toril, so that their captives' gods could not help them (the mulhorandi and untherite gods were just able to send avatars, whose severance from the true deity made quasi-mortal, some of them got killed).
Like all empire based on slavery, it was undone by slave rebellion. The Mulhorandi and Untherite eventually destroyed the Imaskar empire, of which nothing was left.
Just a dead tongue, the Roushoum, and a large badland region, the Raurin desert.
Imaskar is supposed to mean "artificer" in Roushoum. And indeed, they crafted lots of stuff. The Brown Dragons are their creation. The Phaerimm too. As well as lots of powerful-yet-ursed artifacts that lies in the dust of the Raurin desert.