The drow, or dark elves, are an evil offshoot of the elven race, created thousands of years ago by Lolth in a fit of jealousy. Following the Elf Wars, they were imprisoned in Doth'Mordun, a deep valley in the frozen wastes at the bottom of the world, bordered by impossibly tall mountains and sealed by powerful magic.
Recently, the drow made a major attempt to escape. The linchpin in their plot was a large sliver of the Negative Energy Plane, which they were somehow able to pull into the material plane within Doth'Mordun. In addition, agents serving Lolth from outside the prison managed to locate and destroy two of the five keys powering the seal. With the barrier weakened and a piece of the negative energy plane as a power source, the drow were able to "shift" several large chunks of land outside of their prison and into the world, with the drow who had been shifted with them opening stable portals into the prison. Meanwhile, a number of evil deities and arch-demons allied with Lolth and, gathering a a large force of their minions, assaulted the forces of good stationed outside the main prison gate. A band of heroes was then called upon by Corellon Larethian himself to enter one of these portals and shut down the drow operations, which they did successfully by destroying the piece of the negative energy plane. Boccob cast a new spell to strengthen the seal, and Lolth's allies outside the gate were largely defeated and scattered.
Though a number of drow managed to escape their prison before the portals were shut down, it is unknown exactly how many escaped, or have survived in the past thirty years, as very few have been seen since.