True Polymorph Shenanigans

[TheTeaMustFlow]

Step 1: True Polymorph a rock into a creature. Any creature will do, but one that will sit still and cooperate with crazy magical experiments is preferable, so let's say a Mage.

Step 2: Take a Long Rest.

Step 3: True Polymorph the Mage into an Archmage. (Actually, anything that can cast True Polymorph will do, but AFAIK Archmage is the only one CR20 or below, excepting a Metallic Dragon using Change Shape into an Archmage.)

Step 4: Take another Long Rest. While doing so, show the newly-minted Archmage your spellbook, and have them prepare True Polymorph.

Step 5: You and the Archmage both True Polymorph a rock into a creature...

Now, doubtless this has occurred to others before, and the method has a couple of flaws, primarily that it requires the beings created to cooperate with you when they are not magically compelled to do so. But I think the fact that any caster with access to True Polymorph can potentially create unlimited permanent CR20 or below beings at no cost bears pointing out.

[Gastronomie]

Wait, how can you polymorph a Mage into an Archmage? Creatures with CR can't be polymorphed into stuff with a higher CR.

But well, yeah, you could nonetheless turn giant monsters into solid statues of pure gold to troll the hell out of the markets. Purple Worm is Gargantuan and has only a +4 WIS bonus, and if you're Level 17 or higher your spellcasting DC should be around 19-ish, so chances are it wouldn't be able to resist the effect. Dragon Turtle is +6, as another option. This of course assumes the setting is low-magic and high-level casters appear only once in several centuries or something like that.

You could also gradually build a powerful army. Young Silver Dragons will be the main attack force. Mind Flayer Arcanists, Glabrezu, Drow Priestess of Lolth and stuff like that can open up magical opprotunities. Nycaloth can give you unlimited Dispel Magic, and Gray Slaad gives you unlimited Major Image, if you ever need them. Having a Couatl or two in each squad can help as the "party Cleric". Green Hags forming a Coven can use Scrying, which is crucial for any magical army.

Go dominate the world. GO FOR IT!!