Listen her fellow DMs, I'm sure you all like your fearsome dragons and vile lichs, but there's a a new type of villian I haven't seen that's sure to rock your players' metaphorical socks off.  The Archdruid, blending the interchangeable abilities of being a powerful spellcaster with control over nature and the elements makes them a force to be reckoned with.

First and foremost, the most underutilized spell, Antipathy/Sympathy.  Sure, it's niche in that it can only hit a specific race, but then consider that could easily force one or two players to come right towards the big bad.  Really good for a compelling reveal where even players that are fearful will be forced to come face to face with deadly touch spells like Contagion or Blight unless an ally saves them.  Or just prevent the Ancient Green Dragon in the forest from being meddlesome in their plans, turning it into a plot necessity and potential weakness.

Secondly, and the most devious and OP spell in your villains arsenal is Animal Shapes.  Turn your petty Twig Blight army into a raging stampede of bulls, or many, many bears and force your players to retreat or use their deadliest spells to deal with a massive swarm of fodder.  Got some tiny creatures?  How about you have them swarm the party and then have Animal Shapes cast on them, suddenly exploding into a cascade of vicious ursines, crushing the players before they even get a chance.

Did I mention battlefield manipulation?  Good like fighting the Druid on his own turf with hurricane force winds, torrential winds, and unbearable heat from Control Weather.  How about just looking at a town and casting Storm of Vengeance.  Feeling extra evil creative?  Use Mirage Arcane and make a square mile of land a deathtrap, and put a Druid Grove in there for good measure.  Also Whirlwind, Earthquake, Tsunami...Ya know, cause why not have a villian that can destroy entire cities every once and a while.

Okay, now to some more general stuff.  Firstly, any enemy with access to healing spells is no joke.  Nothing is more frustrating then slowly pinging the enemy down, only for them to Heal 70 HP after they Regenerate and stand back up.  If you use the Druid straight from Volo's they also have acsess to CR 6 wild shapes, so if you want to be extra frustrating powerful, make them a level 20 druid so they can change as much as they want.  However, any of the Circles are just as scary.  Land gets powerful extra spells, my particular favorite being the Grassland circle for spells like Invisibility and Dream or Underdark for Cloudkill(especially since Land druids are immune to poison) and Greater Invisibility.  The Circle of Dreams can persue the players tirelessly, becuase they can literally do it in their sleep; usually better with another ally that they can heal and teleport with their abilities.  The Circle of the Shepard makes every battle an ordeal, with powerful animals that are summoned even after the villan falls (and probably gets back up and casts Plane Shift in the resulting chaos).

Lastly is decking them out with magic items.  If you think wildshape is scary, how about a Giant Constrictor Snake with a Belt of Giant Strength.  Make them extra terrible with a Helm of Brilliance, because being able to cast Prismatic Spray is totally fine.  How about some Armor, +3 and a Shield, +3?  Need a plot item that the players need to keep out of the villain's hands?  Simple, it's the Staff of the Woodlands, because with it the Druid could create an infinite Awakened Tree army if they have enough time;  the staff can cast Awaken, and it doesn't require the expensive material component from casting it normally.

So yeah, that's why your next villain should be an Archdruid.

Disclaimer: DO NOT make them level 20 unless you want your players to rip their hair out fighting a Mammoth over and over and over and over and over...

Quick correction: the Archdruid would only be able to wildshape into a mammoth if they were a Circle of the Moon druid first.

This. Is. Perfect. I am currently establishing an evil Druid group as the main villain of my homebrew campaign, and this is absolutely right for the BBEG. Thank you!

Quote from AbyssalSorcererKing >>

Quick correction: the Archdruid would only be able to wildshape into a mammoth if they were a Circle of the Moon druid first.

An NPC or "monster" doesn't follow the restrictions of PC character classes/subclasses. They can do what their stat block says, no more or less, and an Archdruid can absolutely turn into a mammoth:

Change Shape (2/Day). The archdruid magically polymorphs into a beast or elemental with a challenge rating of 6 or less, and can remain in this form for up to 9 hours. The archdruid can choose whether its equipment falls to the ground, melds with its new form, or is worn by the new form. The archdruid reverts to its true form if it dies or falls unconscious. The archdruid can revert to its true form using a bonus action on its turn.

OK. Sorry, I forgot that.

Druids don't need to be evil to be enemies. They just need a solid 'survival of the fittest', and/or 'this human city is a blight upon the land' combined with a solid dose of 'I will do what's necessary - only I have the strength to make the hard choices.'

400 years ago, Greenvalley was a small paradise. The small Slothful River meandered from the mountains, and brought nutrients that fed forests and meadows, bogs and lakes and heath and pasture. The small human settlement was an integrated part of the natural world, and all was as it should be. Old Greenling Mossbeard (a dwarf) was happy.

But as these things go, there were metals underground, and coal, and as demand rose, the small village became a mining town. A dam was built, drowning much of the upper valley, and water was restricted to the lower, ruining the wetlands. Irrigation was built, and trees were cut to raise crops to feed the growing population of the town. 

All the while, Mossbeard tried to persuade the humans away from this folly - and when words failed, he tried to use limited force, to put weight behind the words. 

But humans, blinded by greed, never listen. And so now, Greenling Mossbeard has become something more than a druid. He has become a vengeful defender of nature. He has raised an army of the wild, bears and treants and eagles and wolves and wyverns (yea, he get's wyverns). Mice and rats to destroy the granaries. Flocks of birds to blind archers. Forest goblins and gnolls have joined him, for chaos and plunder alone. Soon, the outlying farms will be the first to fall, and terrified survivors will bring word to the town of the apocalypse of claws that is to come.

May the fittest survive.

I'm a bit bored by the old druid trope of "city bad/wild good".

A different take: as we know in reality, dense cities are better for the natural world than sprawl. They're more energy efficient and economical by a long shot. So the druids are happy to have cities around, and actively help cities develop and grow upward, but will mess you up if you move too close to the frontier. So the druids have built a gigantic wall around a major city, and are not allowing people to leave. Also veganism is law. In this world, some druids who grew up in the city may also not care much about the natural world. They may just inherit the traits and outlook of the local zeitgeist, which could be forward-looking and urbanist.

You could flip Dark Sun on its head. Druid-kings have abused their magic so much that even the cities are overrun with life, choking the space with verdure and evolving terrible kaiju-esque monstrosities. And the druid-kings themselves have existed in animal form for so long that they've forgotten how to be sentient humanoids.

Or a more mysterious plot: a druidic army is moving slowly across the land invading and destroying all in their path. The reason is that they are trying to safeguard a moving target, such as a portal or entity that moves underground like a tectonic plate. The characters could have to work out the pattern in their movement.

Also, there's a weird lack of lore about how druids perceive other planes. A druid may decide that the best way to preserve the natural world is to invade other planes of existence and seed earthly life where it doesn't belong. To folks on other planes, this could be perceived as an attempt to drag the multiverse under the control of primal spirits, which are like Lovecraftian threats to the outsiders. Or you could seize upon the inherent vagueness of the concept of "natural". Druids could decide that the environs of other planes are more "natural" than those of the material plane.

Quote from Gnomarchy >>

I'm a bit bored by the old druid trope of "city bad/wild good".

 

 It's not about that, though - it's something as real and tangible as a ressource war. Humans are stealing our forests to make fields, we're just taking back what's rightfully ours. Druids are, after all, neutral, and don't care whether 'city bad'. 

Anyways your ideas are pretty good, I like 'em =)