**Spoiler Alert for The Lost Mine of Phandelver!!!**

I'm a newbie dm (Running WBtW for about 9 months now smoothly. And I've started running LMoP for another group.

I'll try and keep this short but I took a friend's suggestion and replaced the sorta boring Black Spider for Halia Thornton and making her a changeling. She's a (ex?) adventurer who heard about the riches/forge and wants it for herself. She's spend the last few years earning the trust of the town and learning what she can. To explain away her uses of shape changing, she has the town convinced that they're suffering from something called Phandalin Fever. A mostly harmless fever that results in memory loss or harmless fabricated memories. 

Anyways I have two questions:

1) How do I make her character real? Like where is she from? Why does she want the treasure? Does she know about the Forge, and if so, why does she want it? Maybe she hates Dwarves/the Rockseekers specifically for some reason? 

2) Hoe do I build her mechanically? I like the idea of her having her own full blown character sheet since she actually is an adventurer. If that's inadvisable I'd love to know, and know why. Regardless I want something that is a master of deceit and able to pass deception checks all campaign under the players noses. But I also want something that will really challenge the players in the final battle. I was thinking 3 or 5 Levels of Fey Wanderer Ranger to start since it fits the Changeling background, offers the bonus to Charisma checks, and offers a decent martial prowess. Is this a good start or am I way off? If I went with it, I don't know if I would follow up with more martial. 5 Ranger x 2 Rogue? Or should I consider spellcasting? 5 Ranger x 3 Trickery Cleric, for duplicates, Mirror Image, and healing? 3 Shepherd Druid for summoning so that she has minions? 

Clearly I've never done this before. Please be kind, but any advice is welcome!

If you are looking to flesh out an NPC, I'd suggest trying out a few rolls on the NPC Generator. Take special note to look at the various "plot hooks" that come up, and see if any spark some ideas. As an example, the first thing I landed on was "She is being controlled by an illithid." This might work long-term for a changeling- spy with some minor mind powers. 

As to how to build her, I would shy away from using PC building structure as 5e is not really designed well for PC vs PC interactions. Instead, I would look to kit-bash some monster stat blocks together to get something akin to what you are aiming for. If I were to give a go at your NPC, it would look something like this:

• Step 1: Start with a shapechanger that has the right sort of base powers I am looking for. (Doppelganger)
• Step 2: Add some abilities from other monsters that fill in for some of the missing powers. (Mind Mage)
• Step 3: Adjust HP and defenses, and Save DCs to match needs. If the boss needs minions, add some. (Gnome Squidling, Gremishka, Pest Mascot, or Wretched Sorrowsworn?)

See how that goes for you.

What I'm about to say is more of an alternative suggestion, and not advice on your concept, so feel free to ignore me. In my combined LMoP + DoIP campaign that ended very recently, I overhauled pretty much the entirety of both modules, including the Black Spider. Here's what I did:

At the back of Wave Echo Cave, there is a large cavern with a steep drop-off into the water below. It seems to stretch on for miles, with loud waves reverberating throughout the cave, hence the mine's name. At a specific point, a hundred or so feet before the room's entrance, the Weave (the magical "threads" that permeate the universe) comes together to a "knot." The Weave is stronger as a whole throughout the mine, but at this exact point, it is strongest. The dwarves, gnomes, and wizards of the mine worked together to build a large stone bridge extending into the room, and erected the Spellforge at that point, carving sigils into the forge to further concentrate and amplify the Weave at that point. A brilliantly bright, multi-colored flame sits at the heart of the forge.

I replaced Nezzar with Sassandra Helvar, a slightly insane, obsessive female* drider sorcerer. Before the events of the campaign, she failed a loyalty test (AKA, a Test of Lolth), and was turned into a drider as punishment. Now, she seeks out the Spellforge in an attempt to cure herself. Through a dark arcane ritual, using her spell-draining staff, Sassandra intends to drain and destroy the Spellforge, using all of its available energy to power a wish and turn back into a drow. (She also had a personal connection to one of the players, a drow who escaped from the Underdark. His escape was the reason she failed her loyalty test.)

Why did I make these two changes? Well, in my opinion, both the Black Spider and the Spellforge are rather boring in the base module. They kind of just exist, and the players just happen to come across them. By making the Spellforge a more prominent landmark, and giving the Black Spider a motivation for seeking it out (something beyond ruling the world), I hoped to make it more interesting. And since I combined this module with DoIP, my players were searching for the Spellforge so that they could enchant their equipment and prepare themselves for a fight with a certain titular dragon. If Sassandra succeeded, they would have been ill-prepared for the fight to come.

So yeah, that's what I did. Do what you will with this info.

*yes I know driders are technically genderless. Sassandra, at the very least, WAS female before she transformed

I’ll probably regret asking but why is the thread title “Babies First BBEG,” ?

Quote from Appellion >>

I’ll probably regret asking but why is the thread title “Babies First BBEG,” ?

 I assume it's a joke on the "newbie DM" making his first own BBEG?

Okay, thank you.  I figured it was something simple like that but hated the idea I was missing greater context.

This was great, thanks you! I'm ditching the full PC creator for a NPC generator now. 

When I ran LMoP i buffed the doppelgangers to make them Mini-bosses (one had replaced a bandit chief and the other the newly elected mayor).

I gave them max health (well, if everyone could make it to the session they would have 80 health, if someone couldn't make it then they would have 52 health), the skills of a level 7 rogue (so cunning action, uncanny dodge, sneak attack and evasion) plus 1 legendary actions. First round they would use a legendary action to use Read Thoughts but against every cresture within 60 ft instead of just one. Second round would make it so they would be under the effects of the Dodge action if they kept concentration for Read Thoughts. Third one would give them advantage on attack rolls until the end of their next turn against one creature. They also had the armor and weapons of those they had replaced (as well as proficiency with them) and could do multiattack with them rather than just with their slam attack.

They were supposed to use their position to fight alongside minions too.

I hope that helps.

Edit: Oh, I also had the Black Spider turn into a Drider and alter the Forge of Spells so that it would let him make giant mutant spiders. Maybe the doppelganger wants it to create Mimics and other Shapeshifters loyal to them to infiltrate different settlements and take control of them.

Ooh, building villains is really fun to do and I recommend beginning by going to the "Villains" section of the Dungeon Master's Guide if you have it!
For her motives on why she is being bad, I believe one of the best ways to create a villain's backstory is by making it as bad as they become in present time. You could say Halia was born with many siblings and was typically the least favoured out of all of them, then after a long time she finally decided that she wants to prove herself worthy in her parents eyes (who have long passed away) by strenuously studying and searching for the treasure. As a result, when the PCs finally find the treasure she had already gotten to it first (this can be evidenced if it is a dungeon, many of the traps have been deactivated!). The players can choose whether or not to fight her or settle things in a calmer and easier way.

I do not recommend creating an outright character sheet for a BBEG, however you can certainly use a statblock off Google and rename it to your liking! Editing a monster statblock would be much more efficient and easier to deal with, as it is a lot more easier to look at and comprehend when you're busy dealing with all the other characters' actions.