Concept - I was looking for inspiration on a character I'd like to play and while watching The Grudge with my girlfriend the other night, I thought of a character concept based around Kayako's skill set. A person who focuses on mentally tormenting and terrorizing their enemy, then lunging from the shadows for a quick kill when they're all but broken. I've thought about Shadow Sorcerer since that gives Darkness plus the ability to see in magical darkness, then only taking spells that makes them as mobile as possible and fits this sort of tactic. The issue is I'm not sure what I need to do to make it so it's not a matter of bum-rushing the sorceress catching her trying to sneak to take her out.
Backstory - Her name is Syl. I've got a rough idea of a good one but I'm trying to figure out how to make it so it's not an instance of the backstory being more interesting than the campaign. Like her being in the party seems like a complete mismatch or where it's like "the most interesting thing in your life already happened, why are you here?".
What I have in mind is she is the daughter of a chieftain of a large village. Her parents are tyrannical and oppressive in how they handle things and while she shares some of their values, the differences she secretly has are drastic enough that she genuinely wants to kill them but refuses to do so because "they're my parents" despite the fact that nobody knows what she looks like. The culture is, at birth, you are assigned a task. Adults are required to take that as their profession and do nothing but that, children are required to essentially remain unseen and master the craft they were assigned. This even extends to nobility and royalty. Syl's job is to master the art of politics and has even less freedom than other children. They have classes where they all learn together and can be social with other children who are assigned the same job, even the nobility have this. But because her job is to be a ruler, nobody else would be in her class. Additionally, she has no siblings. There is a one-child rule due to limited resources. Additionally, twins are considered evil in their culture, where the twin that is born second is deemed to merely be a demon in disguise seeking to steal the first's soul and slaughter the family (and possibly the entire village). In every case, the twin born second is taken into the forest and left in front of a cave of a sacred mother bear as sacrifice (the reasoning is unknown, it's simply an ancient practice they uphold out of tradition and fear now).
One day, an assassin sent by the government of a neighboring (and well-hidden) city of elves deep in the forest entered Syl's chambers with the intent on killing her and her parents. Over the centuries, they began viewing the village as a blight on the land and wanted to destroy the leadership so they can take control by emerging from the background as saviors who helped an unfortunate people, not as conquerors. The assassin never went through with killing Syl, her (identical) twin sister. Neither were aware of them having a sister and it went from an assassination mission where the two wound up just getting caught up getting to know each other. Both had always wanted a sister had always longed for the romanticized depiction of the others' type of life (the life of a wandering adventurer and the life of a beloved princess/queen).
From here, I'm unsure of how this should lead into Syl traveling with this party in a faraway land. Yes, she doesn't want the life of being a ruler but she also doesn't want to abandon her people. I'm thinking maybe barbarians are on march and burn down the entire village and kill her people to take the land, which results in Syl and her sister running away and returning to silently pick them off. A few barbarians escaped and maybe a couple elven scouts noticed the clouds of darkness from a distance, but found nothing but dead bodies when they got close enough to investigate. As a result, the land is now considered No Man's Land. The barbarians who escaped spread the legend of a "Shadow of the Ash" while the elven city wound up spreading a warning that fit too well with the barbarian's legend. Maybe now Syl is adventuring while her sister works towards her own goal of leading the assassin's guild/queensguard of this elven city. Nobody knows Syl is the "Shadow of the Ash" and her past is largely a closely-guarded secret.
Skills - I'm wanting Stealth, Intimidation, Deception, and Insight. Her charisma score would still make her good at persuasion but it's not her strong suit in social situations.
Race - I keep debating between these:
Tiefling. The Glasya variant's +1 to Dex may help with AC, plus each of the three spells fit how she fights and give her more to use to keep how she does things fresh. However, the Fierna variant's +1 to Wis and the three spells she gets here fits with the more social side of her character.
Elf (Eladrin ToF). Gives her the ability to be much more mobile due to teleportation 30 feet being a bonus action (Fey Step). But, Spring seems to be the only one that she directly benefits from because it gives her more to aid the party. But, I don't see the value of the other three because they require teleporting into striking range to maximize their worth, which isn't good for sorcerers. The +2 to Dex and proficiency in Perception is also nice.
Half Elf. Dark vision is wasted on a Shadow Sorcerer (unless this doubles it) but having advantage to being charmed and immunity to sleep spells might fit with this character concept. Proficiency in two additional skills might be nice but in terms of what would fit her character, I don't know to pick. The Wood Elf Heritage variant, however, gives that up for adding an extra 5ft to movement plus proficiency in a longsword/shortsword/longbow/shortbow, and the ability to hide even when lightly obscured are all great.
Classes - I've already decided on a shadow sorcerer but I'm debating on multiclassing or just going pure shadow sorcerer.
3 Rogue/17 Shadow Sorcerer. I'm thinking about either Mastermind or Swashbuckler.
6 Rogue/14 Shadow Sorcerer. In addition to the archetypes, a second Expertise plus uncanny dodge seem very useful. Also, most of the spells above 14th level don't seem to appeal to me all that much at face value. Maybe I'm missing something?
3 Fighter/17 Shadow Sorcerer. Having breastplate for extra protection, plus the Defense fighting style, and the Scout archetype seems like a great fit. I don't plan on having her in the front line, but I do want her to be a little more durable.
6 Fighter/14 Shadow Sorcerer. Adding in an extra attack and two feats or ability score is enticing as well.
3 Bard/ 17 Shadow Sorcerer. The College of Whispers archetype seems like it might take the character to the next level as well, plus having more spells that are cast using Charisma.
6 Bard/17 Shadow Sorcerer. Adding in Mantle of Whispers and Font of Inspiration to increase how often I can use Bardic Inspiration, plus a 1d8 instead of a 1d6.
I know this is very long and I want to thank anyone who read some or all of it beforehand. Do any of you have suggestions or thoughts on any of this?