The Black Spider had a head start to Wave Echo Cave. He had time to start excavations for long forgotten items of power as well as set up a formidable defensive line. He was not expecting a battle on two fronts though and ultimately that cost him his freedom. The Cragmaw Bugbears had been either holed up building an offense against the defenses of the Forge or Spells or put to work in their excavations. This allowed the party to slip behind them through mostly unused tunnels and neutralize the threats before they were aware of the incursion.
Nezznar, the Black Spider himself, put up a strong defense with the aid of his magical training but was ultimately captured. His prisoner, Nundro Rockseeker, was freed to reunite with Gundren and carry the news of the death of their brother, Thurden. Sildar Hallwinter was happy to accept custody of Nezznar so he could face his rightful punishment in Neverwinter.
The party now freed of the threat of the Black Spider could now work towards uncovering the Forge of Spells. Clearing the mine of dangerous forces would allow the town of Armstead to again prosper.