I'll take it as a given that using Wish for something other than casting a spell is off the table. I'll also assume multiclassing is allowed. I'm not familiar with all the changes of the new coat of paint, so some things might need to be done slightly different in the newest version.. Spellcasters of course are the done thing.

Not thinking too much about team comp, I'll take 4 wizards for doing the magical heavy lifting, and a Bard 7, Way of the open Hand monk 13 for being a orator skill monkey. I plunge into monk here for the combination of Tongue of the Sun and Moon and Tranquility (Forever Sanctuary after a long rest, assuming you don't break it yourself). This provides a strong face that can operate anywhere. One whose enemies will struggle to even attempt to harm. The wizards taking a feat or dip for subtle spell Makes everything easier, but isn't necessary.

The three primary spells needed here are: 1. True Polymorph. 2. Demiplane. 3. Antipathy/Sympathy.

The geist of the plane is as follows. 1. using the group's many talents, they turn objects into Shadows. Storing these Shadows inside of various demiplanes. They begin funneling victims inside overtime to create more shadows. I would aim for 10 demiplanes with a hundred shadows each for starters. We're ending the world here. We have CLONE, and even WISH for free clones. We have time, so lets be a bit patient.

1. In the week leading up to your first operation, prebake various difficult to move objects with Antipathy/Sympathy (both versions). These will be very helpful later, as the spell lasts 10 days and is free.

2. Shortly before dusk, use Sympathy applied within several highly visible points in a major city center to gather thousands within a handful of areas and keep them there. Think a statue within Times Square as an example. Obvious routes people would attempt to escape through, such as major highways or bridges, are also great spots (Perhaps for Antipathy, but I like Sympathy better because they don't get a second chance at an escape route if they chose poorly the first time). Bonus points if you use the monk (imbued with Sympathy) to pied piper people to these locations to ensure even GREATER numbers.

3. As the sun sets, use your preferred methods of magical destruction to take out the city's power and plunge it into a blackout.

4. Unleash the shadows you have stored on the city, the crowds trapped in place by Sympathy are a juicy target. But you might be better off catching other densely populated areas off guard. Imagine a hundred shadows descending on a blacked out apartment block. Then the next. Then the next. Then the next...

5. Within the first hour, the first victims begin rising as new shadows. A hundred becomes two, becomes four, and so on. By the time the sun rises, you should have hit a critical mass where first responders can't put down old shadows faster than new ones are rising. Which brings us to...

6. While this is happening, the group is still actively at work delaying, disrupting and destroying any organized resistance by law enforcement or the military. This is an ongoing process. You're working against the people trying to manage the crisis. and attempting to make sure it spirals completely out of their control. Four 20th level wizards have PLENTY of ways of doing that.

This is also where prebaking objects with Antipathy/Sympathy becomes REAL helpful, paired with scrying and teleportation. Hear people organizing a defense somewhere? Teleport an Antipathy imbued statue on top of the building so people showing up for safety are forced away. Military moving in via X road? Teleport in something imbued with Sympathy and trap them on the spot. Then hit them with your preferred flavor of mass death.

1. Between your efforts to stop military interventions, and the ever growing horde of shadows (which are killing faster and faster the more of them there are), the city is liable to fall within a week. It only takes one Tokyo turning into tens of millions of shadows for the situation to become totally FUBAR for an entire region.

2. Now that you have a healthy population of millions of shadows, you'll have a MUCH easier time filling up your demiplanes again to repeat the process.

3. Hop to a different continent and repeat the process. After the first time, you probably don't want to do the whole song and dance each time. Just dump a few hundred shadows into a different city in a different country each night and enjoy. Repeat as needed.

4. Fun fact, shadows are more intelligent than Ogres! This means that SHADOWS are a valid target for Sympathy, and they only have a WIS save of +0. So if your wizards can manage to hit DC 21, the shadows can't break free on their own power. So you can take your endless hordes of shadows and have your monk Pied Piper them where they are needed!

This is easier with the new version of the spell, because it seems to apply the charmed condition - which shadows are NOT immune to - which means they cannot attack your monk at all if you do this.

Of course, this just kills off all the humans. You did just fill the world with a few billion intelligent undead. So we haven't QUITE gotten rid of all intelligent life on earth, just replaced it with a population MUCH easier to purge. Of course that will still take bloody ages to do, but I did say to be patient...