Thanks for all the thoughts, folks. Especially Mouseferatu. I hate that too. 
Yeah, normally I wouldn't spend too much time on crunch for an NPC, but this one is the twin sister of one of the PCs, and the PC is multi'd as a warlock herself, so I fully expect to have to answer detailed questions about this NPCs craft and background (so that the PCs can plan appropriately when the time comes to confront her). Also, I'm tentatively setting this NPC up as a campaign-arc BBEG, so I expect to have to level her up a few times, adjusting powers each time -- I want to be consistent.
So Lolth is a demon, right? I suppose that means that the Dark pact is the closest fit. I guess I'll have to go pick up the FRPG now.
Does it say somewhere specifically that demons don't do pacts? Where is that? The NPC is Tiefling, so I could always give her Infernal Pact for the racial heritage, and just make the demon-consorting a hobby as opposed to a calling.

Yeah, normally I wouldn't spend too much time on crunch for an NPC, but this one is the twin sister of one of the PCs, and the PC is multi'd as a warlock herself, so I fully expect to have to answer detailed questions about this NPCs craft and background (so that the PCs can plan appropriately when the time comes to confront her). Also, I'm tentatively setting this NPC up as a campaign-arc BBEG, so I expect to have to level her up a few times, adjusting powers each time -- I want to be consistent.
So Lolth is a demon, right? I suppose that means that the Dark pact is the closest fit. I guess I'll have to go pick up the FRPG now.

Does it say somewhere specifically that demons don't do pacts? Where is that? The NPC is Tiefling, so I could always give her Infernal Pact for the racial heritage, and just make the demon-consorting a hobby as opposed to a calling.