"Why take for free what you can cause pain to achieve?"
Alignment: Chaotic Evil
Domains: Darkness, Trickery, Deceit
Followers: Lolth's Web; Priests are Arachne
The Spider Queen
Lolth, the Spider Queen, rules over the Underdark. Her followers, mostly Drow, both fear and love her, depending on which one you speak to. Lolth is inherently self-interested and will follow whatever path benefits her the most, regardless of consequence.
For many milennia, Lolth ruled over the Underdark, the Faedark, and the Shadowfell. When he was kicked out of the Faewild by the Greater Fae, Loth ceded her control of the Faedark to Ystrom, the Change of Seasons. By that time, she had already turned her eye towards the Material Plane.
Unfortunately for her, the Council of Three decided not to allow her to invade the Material Plane. They met her in the Shadowfell for an epic battle. During the battle, the powers of the Council, Lolth, and the Shadowfell somehow coalesced around a Voidsoul Genasi and created a new deity called Jenova. Jenova aided the Council of Three in defeating Lolth and sent her scurrying back to the Underdark.
The Underdark Wars
This set off a thirty-year conflict called the Underdark Wars. Initially, Lolth vowed to take back the Shadowfell and destroy Jenova. At certain points, Jenova vowed to eradicate Lolth altogether and free the Underdark from her control. Jenova offered free refuge for any Drow seeking to escape Lolth's Web, and many of them took her up on her offer. This enraged Lolth.
After thirty years of fighting, Jenova won a decisive victory, though not enough to eradicate Lolth or take the Underdark. It was, however, enough to make Lolth stop trying to take the Shadowfell, and so the outright wars ended. A cold hostility sat between the two deities, and the two realms, for centuries.
The Harbinger Coup and Cyric
Lolth was approached by Gramorn Duskfell, an emissary of the one called Cyric. Cyric offered to get rid of Jenova and return all of her children to the Spider Queen; Lolth would also regain the Shadowfell. All she had to do was offer troops, intel, and aid to the Harbingers in their attempt to usurp the throne. Lolth agreed, and when the Harbinger Coup took place, Jenova was entrapped in Cyric's gem prison. Lolth controlled the Shadowfell for a brief period, until the Sandstriders removed the Harbingers from power and freed Jenova, who summarily removed Lolth from her realm.
Astral Civil War
Lolth felt betrayed by Cyric. He had not come to her aid or helped her in any way, other than to temporarily hold Jenova. Therefore, when the Astral Civil War broke out, Lolth joined Kord's Coalition - not as a matter of principle, but out of spite for the one who had betrayed her.
Lolth was not invited to join the new Pantheon of Gods. Instead, she returned to the Underdark. With Jenova taking the mantle of the Raven Queen, Lolth did not dare to attempt to retake the Shadowfell.