Driders

When a drow shows great promise, Lolth summons it to the Demonweb Pits for a test of faith and strength. Those that pass the test rise higher in the Spider Queen’s favor. Those that fail are transformed into driders — a horrid hybrid of a drow and a giant spider that serves as a living reminder of Lolth’s power. Only drow can be turned into driders, and the power to create these creatures resides with Lolth alone.
Drow Outcasts Much confusion and misinformation exists about driders among non-drow, but all dark elves know exactly what driders are: failures. They have either fared badly in Lolth’s test or displeased her in some other way. 
Once its transformation has taken hold, a newly created drider is shunned by its house and exiled from the community, with nothing but a few meager supplies and its knowledge of the Underdark to protect it. Drow congregate to throw stones at the still-dazed creature and drive it into the tunnels beyond the city’s environs. If it’s unlucky, it’s attacked by a roper, a carrion crawler, or another drider. If it’s lucky, the new drider finds a safe place to hide while its wounds heal.
So begins a drider’s life in exile. Another widespread misunderstanding about driders is that they serve the drow as pickets, elite troops, or even suicide squads. They do none of those things. They are despised outcasts who live on the fringes of drow territory. Even though drow revile driders, they don’t kill them, because a drider’s punishment is to live a long life in wretchedness. Killing one would cut short Lolth’s judgment and possibly earn the same sentence for the perpetrator.
Driders that survive for a long time can become accomplished hunters and navigators in the Underdark. Nothing will reopen the doors of drow society to them, but sometimes a drider can find a place in another community. Someone who needs a guide through the Underdark might not find a better one than a centuries-old drider that has faced every hazard those tunnels hold.