- A "Masked Traitor" is the term for a priestess of Lolth who secretly serves Vhaeraun. When speaking to Halisstra, Seyll claims to have been a priestess of Lolth, but the Auzkovyns are a notable organization of Vhaeraun worshippers. Also, her brother Szorak later appears in Sacrifice of the Widow, and he's definitely a priest of Vhaeraun as well as very bitter about women. His sister defecting to Eilistraee might have helped aggravate that, and Seyll could have refrained from mentioning she "really" worshiped Vhaeraun so that she'd have a better chance of connecting with Halisstra.
WMG / War of the Spider Queen