The Church of Lolth

The Priestess Trials
It’s a four stage process:

1) a casual blood sacrifice to Lolth. Choose someone, bring them to church, stab them. Just casually. As you do. The most “common” sort of sacrifice is either a slave or a low ranking male. Some aspirants splurge and grab a high ranking woman, or a consort. Famously, the Matron of House Nailo kidnapped and sacrificed her own mother to become a Priestess.

2) A written test. A large part of it is for a focus on ceremony and emulation of the past. The form of the thing isn’t strictly improved: if you’re looking to nail the outcome, five years of training is enough.

3) Combat trials: fight someone. You win at first blood, and get bonus points if it’s to the death.

4) Drink a potion, visit Lolth in the Abyss, and have her personally judge your worth. Once those women are up to meet Lolth though, all bets are off. Some classes are killed to the last, others turn into Driders, and once in a blue moon every aspirant is chosen as a Priestess.

Note: only those who pass the first three trials are permitted to attempt the fourth trial. The first three stages are meant to allow the worthiest to rise to the top. By its nature, the challenge of blades allows less than half to slip through, and your blood sacrifice is judged on esoteric and subjective qualities. I’d say that, with the trials held one every decade, only two dozen, or less than two percent, of potentials make it to the final stage of the process.

Draegloths
Survival is seen as proof of strength, and some Drow even achieve a greater form of proof in the form of the conception of a Draegloth.

This is the cultural detail that House Cithreth leverages to not only remain politically relevant, but to maintain their alliance with the strongest House and the Church in the city: Matron Thiala, as I’ve mentioned, has birthed several Draegloths. This is proof of her own personal strength (as she is conceiving with a barely sentient and mostly magical engine of destruction and annihilation) and her favor with Lolth for having survived these ordeals.