Originally Posted by
DeTess
I don't disagree. You can have a perfectly fine game without a backstory (one of the funnest campaigns I've been a player in had a collective backstory for the party of: 'you where all travelling to this city on this boat, but then the boat sank and you washed up on the shore far from the city. You still want to go there though.'), but I've found that in a less combat-focused game, backstories can be really useful in making players feel like they're part of a world and in getting them invested in the world.
edit: What I'm trying to say is this: backstories are a tool, and depending on the game they can be really useful, not at all useful, or anywhere in between, and it's not really helpful to claim one thing or another about them as if it's an absolute that's always the case.
An entity as powerful as Raven is apparently can't stay completely unknown. There's going to be traces of her all through the last decades, and I'd use that to introduce her to the PC's. Have them come across traces or stories about her. To use an example from that campaign involving the dragon god-emperor; we only encountered him face-to-face in the last 7 sessions or so from a 2-year long weekly campaign. He wasn't even on the same world as us for the first 8 levels and 20-odd sessions. However,w e constantly encountered little traces of his past presence. Shrines dedicated to him, amulets that where once held by his followers, a doomsday cult centered on his return (they where right eventually, but got the date wrong), that sort of stuff. So by the time he arrived, it wasn't much of a surprise to the players, and he immediately felt like credible threat because of what we'd already learned about him. A lot of that was also just random window-dressing to dungeons and encounters focused on other things, but because several players where just naturally curious we'd managed to drag quite a bit of his story into the open already.
All this is a long-winded way of saying that when the PC's find out about the BBEG they should already have heard of her, and seen traces of her existence and influence before that moment. Since the PC's already know, I reckon they're probably more likely to follow up on subtle clues about her involvement as well. If you provide some initial clues and give them the opportunity to explore and learn more about Raven, even if you don't immediately confirm her involvement in the main plot, the players should feel like they know their enemy already, and that will make her feel like an active and credible threat, and a good villain.
Based on your descriptions, Rachelle feels like an obvious first step. Byron, Acel and Sarvus all have significant support networks that makes them daunting prospects to tackle, while Rachelle seems relatively isolated. Depending on what the PC's know of them, they might actually meet Sarvus first (he seems like he could put up a front of being more decent than he really is), but he only functions as a quest-giver of sorts. He might even put them on Rachelle's trail, thinking he's getting her some new specimens to experiment on or whatever.
In the context of the 'starting a war' plot, Rachelle's group could be presented as terrorists who are ostensibly aligned with one of the major factions, but are really just trying to start a war by doing horrible stuff in name off a group that's not actually involved with them.