Source: Endless Night, pg(s). 51
Mardeis Parastric is not just the daughter of the powerful matron Astranovidova Parastric, but is also her bodyguard and advertisement—Mardeis is not a drow like her mother, but a fleshwarped creation known as a drider.
House Parastric, the noble house to which Mardeis belongs, are worshippers of Haagenti, the demon lord of alchemy and change, and thus specialise in the creation of monstrous beings that serve the drow. So dedicated to Haagenti was Mardeis's mother that she had Mardeis turned into a fleshwarped creature. Now she travels with Mardeis using her as both a bodyguard and as a demonstration of House Parastric's fleshwarped creatures, which they sell to other drow houses.