Telantiwar was an ancient drow city.
When the drow first descended into the Underdark, they invaded and conquered the Dwarven city of Bhaerynden and renamed it Telantiwar. Several enormous magical explosions during a civil war among the drow eventually led to the collapse of the cavern of Bhaerynden. The cave-in buried and crushed the city, forming the Great Rift.
Gold dwarves believe Moradin destroyed Telantiwar with a blow of his axe, but scholars in other races believe the drow weakened the roof of the cavern through tunneling and relying on magic to support the weight of the ceiling.