Recently i've been going through various spells and items that i'd like some NPCs to have/use.
They all have requirements that no average person could achieve.

As the DM, can i have something in my world occur and just say; "they were able to because of..."
I usually hear the the DM can do whatever they want, does that also encompass the creation of NPCs?.

I'd also like to have "evil" NPCs, like a lich, that are nice/reasonable/chill.

for exampe; a lich that sells a clone to a rich person in exchange for materials and souls.

So, as an example, i could have a murdered man be remade into a lich by a bored Demon Lord?
Course there'd probably be a price- X amount of souls every month/year, to make it worth the Lords time.

If it's part of a story you can do anything you want.

As a DM, you would normally only have a murdered man be turned into a lich by a bored Demon Lord if the lich and the Demon Lord figured into the adventure in some way. If the price is souls, then maybe the party stumbles onto this deal by seeing the lich's servants harvest bodies and bring them to him or something.

I'm not sure what you mean by items having requirements a "normal person" couldn't achieve. There's nothing that says NPCs have to be "normal." You can make an NPC that is a 10th level Warlock if you want, etc.

You can totally do all that.

Yea, i just feel like i have to build every NPC.
I was planning on running the Lost Mine of Phandelver but didn't, because i don't know what the town is like. Or the personalities of every single NPC. I'm not even sure of the population.

I feel like i'd be a bad DM for not knowing everything.

You are as free as your imagination.

Take an adventure or setting and change what you want.

Create your own world.

Create a billion universes.

Make an NPC that is an all-seeing giant eyeball the size of a thousand galaxies or maybe make a mouse that causes a tiny brief blackhole when it hiccups.

Whatever you want.

I have a horrible memory, especially when using something that I haven't come up with, so I often modify villages/NPCs to help me remember them. Makes it easier to make them unique as well.

My group doesn't remember most NPCs, but the ones they do remember are the NPCs that took on a life of their own. And now I suddenly forgot all their names...

Anyways, a good example was a random merchant who eventually became the go to guy for buying exotic or magic items. A loud personality, a little shady, and eventually discovered to be a werewolf, but still the preferred merchant to deal with.

Another one was a random goblin that became the sworn nemesis of the rogue (and the player). Due to poor rolls for the player and great for the goblin, as well as some poor tactical choices, the little guy managed to briefly knock out the rogue and stole most of his money. It lead to some hilarious moments, but unfortunately, it was near the end of the campaign. I tease the player that although the group won the campaign, he lost, lol.

It’s okay to not know everything. A lot of stuff you can just wing it.

Quote from kootranova1 >>

I feel like i'd be a bad DM for not knowing everything.

Don't let that stop you. I once named a major villain after the brand of pencil that I was using at the time. Jensen the werebear warlord lived on for years!

I feel like i'd be a bad DM for not knowing everything.

You can't know everything. Even about a world you made up yourself. You can't know the name of every single NPC in a city... there are tens of thousands of NPCs. Most of whom the players will never meet. The trick is to make up a few NPCs you think they are likely or certain to meet, and sketch them out a little bit (name, job, age, race, primary motivation, maybe alignment if you like to use that sort of thing, which I do). And then have a list of names somewhere that you can pull from in case you need one, like a town guard standing at the gate. Most of the time the characters will walk past him. Sometimes, they will want to talk to him. That's when you consult your list of names and find one to use (and cross it off so you don't use it again).

You have as much freedom as you can want. However be aware of your player's expectations - if you go too far in a direction they don't like they'll be out the door to play with someone else.

You only need to know as much as the PCs need and is relevant to the story. The players should not expect you to know everything.

About half of the time that social interactions happen I find that I am just improvising as a DM.