This founding / unifying entity can be almost anything or anyone: a person, a ghost, an organization, a sentient magic item, the manifestations of a culture or even a talking stick. You can even go with struggles with non-intelligent forces such as 'nature' , 'bad luck', or 'one's inner demons'.
2. So What? What are they up to? What do they want? Why do they want it?
Your chosen entity must have a reason for why wants to do stuff. It wants something.
3. What Is Next?: Where is it going? Why don't they have it yet? What could stop them and how?
This part is key. These solutions must be TERRIBLE / cannot be nice else - lest you have no story and no 'use' for the heroes.
Most villains are active (they make stuff happen) where heroes tend to be passive and keep the peace ('take action when they see a problem'). Player's characters are often the same design. Happy with their lives as a farmer, apprentice, town guard or ship-hand, they see no need to expand their horizons. This 'Plus One Problem' must be so impactful that both the player and their character will readily abandon their life of peace and safety. This is your key job as a game master, more important than any other job: the problem you make must have enough threat to make the adventure worth it.
You want to see an example? Yes. Yes of course. Alright, let us give you Grendigg Gruelz, a grumbling goblin. He that arch-villain intent on ruining the world as we know it and he only has a few hit points to start. Let us watch his origin story.
Assuming we are using 5e D&D you know that goblins are small, greenish and sneaky. Those of their 'race' like hobgoblins see them as cannon fodder - and are kicked around by their mutant-huge cousins the bugbears. Goblins are listed as neutral evil - so they cannot even trust other goblins. These creatures tend to live in continual fear - and constantly in danger of violent death. In short: the life of a short goblin can be... solitary, poor, nasty, brutish, and short. Grendigg Gruelz is no exception in this.
2. So What: How does Grendigg rise up?
Looking at lore from Volo's Guide (p. 42): Grendigg is a booyahg. This means he managed to steal-borrow a spell book from a hobgoblin war-caster / superior. Given a goblin exceptional intelligence, he realizes spell-use is a ticket out of racial strife.
Assuming Grendigg improves, he gains an apprentice stat block (CR 1/2) - this also gives him ritual spells. Seeing how Magic Missile requires a group of casters so he starts training others. Since goblins tend to be nasty (culturally), he needs to know who he can trust. Grendigg needs access to one of these: Zone of Truth, a truth serum or regular (ab)use of the Friends cantrip. Getting a TruthZone is easy enough with a cult fanatic - but can he trust the cult leader? A hag coven could also provide spells, but at what cost? If you use the Friends cantrip you make many enemies as you go. It might work better to interrogate all wizard-applicants while they are intoxicated ('drunk'), giving Grandigg advantage on intimidation / persuasion checks. It looks like a combination of truth serum and a lavish open-bar policy is the way to go.
Over time, Grendigg (& his most trusted circle) interrogate vast numbers of goblins - seeking both those smart enough to cast wizards spells and reliable enough to be trustworthy.
The following sets of twists are possible so far:
There are more, but these are pressure points to consider as we go forward into Phase Three.
What's Next: What will Grendigg do?
To keep this story going we must assume Grendigg has his magic-use (wizard) abilities and some of his guild or clan of lesser wizards have survived. We must also assume he has a strong motive to destroy friends, relatives, supporters &/or patrons of the player's characters. This entire encounter can be as simple as 'killing the hobgoblin's apprentice in room 12)a.' to as complex as 'mad genius plots to take over the world and only YOU can stop him!'
A 'plot' is any strong premise or rationale for where the story should go. It stands to reason that a 'plot twist' is any premise or rationale that also makes sense - but it happens later on... and is a bit of a surprise. Thus, this goblin wizard group could either 'lead' or 'twist' (later on) with any of the following:
The Big Trick: provide hints and guidance that each intro-plot ('premise') or add-on plot ('twist') from the start. Then, when players figure it out, reward them for paying attention. When players don't figure it out, have an NPC remind them of the clues they had and reward them for playing along. This creates feelings of inclusion, verisimilitude and plot & character consistency.
This game-story 3 +1 set ('How So / So What / What's Next' + plot-pressure point-twist) is all any game-master should need. This is the format used throughout this website. Any character exists within this 'where from / where @ / where going' timeline. Every variable introduced at any time is a twist of some kind that forces meaning upon your game-story. These twists are unknowns that guarantee your game is not a 'sandbox' nor is it a 'railroad'. No one knows what each of these power-shifts will do - but something has to happen, even if that means the whole story falls apart. It is a game after all.
This device is not enough for the complexities of story-development, script writing, video game design nor book planning. It is designed to help gamers come up with something more nuanced than a dungeon-grind raided by murder-hoboes. Even if your Dungeon Master goes back to drawing rectangles on graph paper and filling them with CR-matching encounters, these simplistic plots will provide both depth and entertainment to make your night's gameplay memorable and fun.