Everyone knows what's coming with a Green dragon... or do they?

Adventurers are heavily prepared to fight dragons. Why? Dragons live for centuries with low-mobility 'lairs' - thousands of stories exist. What with creatures immune / resist poisons, Green dragons are even at more disadvantage. How do you make this creature impressive? Here are three tricks:

Making That Poison Hit... Harder.

There are many kinds of poison in the DM's guide (page 257) as well as other magic items &/or monsters that use poison in other sources. Note the different effects (like sleep, paralysis, damage-over-time and delayed onset) but different kinds (contact, ingested, inhaled or injury). Would any Green dragon (intelligent, resourceful and many followers) master the alchemical techniques to transform one such poison to another type? You bet your dragon-hide booties they would. Could they supplement their breath weapon for far greater effect? Yes - and this is how.

• Essence of Ether: a special blow-tube / cloud mixer (un)common magic item allowing a Green dragon to add Ether Essence poison ( DC 15 con save) or become unconscious for eight hours in addition to the usual breath weapon damages. Those asleep can be taken as slave-captive-ransomed or easily slain by simply breathing again
• Giant Insects Poison! Wasp / Spider: Both Giant Wasp and Giant Spider poisons do not kill the target but, at zero hit points, cause paralysis (& poisoned condition) for an hour. This is a great way for Green dragons to claim, collect & gain 'followers': those covenant-contracts that are notoriously make up their treasure hoard. An alchemist convert-mixing some condensed-distilled Green dragon poison or Dungeon Master could easily justify a custom spell or magic item transforming their breath weapon to have this effect outright.
• Truth Serum: Using magic or alchemy to convert distilled Green drake poisons would have much better than a DC 11 save (older than 'wyrmling'). Since most Greens have treasure consisting of sentient beings (typically humanoids), making contracts under pure-truth conditions is extremely valuable. One could also rule that each dose adds disadvantage and possibly levels of exhaustion.

Mixology: Venomous Tricks & Tactics

A cloud-mist poison breath weapon is unique to Green dragons. There are specific techniques one can use in combination with other situations that make this powerful - especially for unsuspecting characters of players.

• Dreaming Sleepers: Dragons with access to the Dream spell can really make Ether &/or Giant Insect poisons work wonders, directly accessing their newly-found followers' minds.
• Controlled Misting: If terrain is foggy and misty targets will not know what hit them nor which way to run when they figure out they are under a gas-attack. This works especially well if the mist is of the same colour as the noxious breath. This pre-fogged area also eliminates most line-of-sight return-fire. Green dragons will do this above mazes, trenches or gorges exposed to descending mists.
• Undead Buffers: Not known for their necromancy, Green dragons can still collect lesser undead very easily. Using zombies or animated-skeletons as shock troops the dragons can more-safely breathe from above or from a well-protected hole. Any more loyal & immune to poison creatures also work well - golems, elementals, animated objects and (if the dragon likes to live dangerously) most fiends.
• Toxic Slaves: The more poison-reliant a monster is the safer it is to a Green dragon. Obviously a Purple Worm is a bit of a disaster - not only does it do massive damage without its poison, but it also leaves underground passageways to and from any lair and is as smart as a sack of bricks ('will NOT follow orders'). Many other smaller poisonous creature will do fine.
• Armed & Noxious: Giving lesser and otherwise expendable followers, suck as kobolds, access to Purple Worm poison or condensed-distilled Green dragon poison works wonders for terrifying invaders. Imagine a scratch from any trivial creature that adds an extra forty damage.

Spells: So Many Choices!

Green dragons are notoriously intelligent with a lawful nature for plotting, scheming and planning over centuries. They would choose spells with the greatest impact over the most time. Consider ancient dragons have access up to 7th level spells (CR / 3).

• Ritual Spells: Assuming a magical, genius dragon can learn basic wizard-apprentiship skills over a few centuries is not a stretch. This not only makes those leftover spell-books useful it gives them potential access to eighteen more spells - typically cast at their spell-casting level. Note how Gentle Repose allows a Green dragon to Raise or Reincarnate a humanoid later (valuable if they are a treasured follower-servant) but Contact Other Plane would allow a dragon a daily influx of extremely valuable information down through the centuries.
• Reincarnate: Note that dragons cast spells without need for components. Recalling a follower obliterated via Disintegration-style magic is extremely powerful. Imagine extremely old humanoids of great power with youth restored - in some random body, sure - but young again. This also ensures Green dragon 'treasure' doesn't biodegrade and lose value due to time's passage.
• Simulacrum: Casting spells without components means this spell is 'free'. Having a copy of a high level caster is like a self-casting scroll that also thinks for itself. Also: this sentience could be transferred into a sentient magic item.
• Plane Shift: This allows the dragon to leave when faced with otherwise overwhelming odds. They can also build their lairs on other planes. It takes up to eight creatures to any plane, so Greens would go on runs to pick up and drop off treasured followers and powerful 'loot'-slaves. Any ancient Green with this spell has multi-planar influence.
• Person-oriented magic - Charms & Divinations: In building up their hoard-loot of sentient people, making reliable, trustworthy contracts is absolutely key. Tripling up for their inscription would involve a charm, a suggestion, and truth compulsion from either poison (above) or spell. Such a person would be checked first with magic detection. If a Green 'owns' a troupe of Dragon Slayers (for public relations stunts &/or for hunting the dragon-competition) Mind Reading is important. Reading surface thoughts (no saving throw) allows the caster to know if promises are being made just to keep everyone happy - and it also checks to see if mind-protection exists. Also, detecting thoughts may be a cheap and easy way to veto Glibness.
• Suggestion - En Masse: Using Mass Suggestion gets up to 12 at once and it lasts for the day without concentration. It has a dreaded 'wisdom' based saving throw, so it might be best to cast this whilst the recipients are under the influence of a wisdom-breaking poison (if one exists). You might allow a dragon to have this as a 7th level spell slot for a 10 day duration (optionally 30 day for an 8th level slot... or even 'year and a day' duration if you give your 22 hit dice creature a 9th level slot).
• Find Familiar: Greens with regional effects can see and hear via any rodent or bird - it doesn't matter if they happen to be celestial, fiendish or fey. They may also want to magically talk to the animals and tell such spy-cams where to go. You can imagine dragons specializing in such spells to own aviaries and collect families (groups or 'labours') of moles to serve as... moles.

You will see that Green dragons do not choose 'boom' spells like Fireball. They have executive-management spells. They may also like information spells (like scrying and lore-research) if they wanted to know more about their entourage-ensemble of servant-slaves.

Oh, The Wicked Wonders We Weave - Magic Items For Green Dragons.

Sinfully intelligent, Green dragon build hoards rather than collecting them, investment in sentient creatures means access to vast networks to gather components, recipe-pattern-blueprints and devices to enchant nearly any magic item they need. These magic items are more than empowering bribes - if designed correctly, these can mark and control the location and actions of any servant-slave.

• Sentient / 'Intelligent' Magical Items: No edition is clear on the cost, process, value nor duration of sentient magic items. To Green dragons, these items are nearly priceless for keeping servants on track, spying, reconnaissance, networking - and easily findable with locate magics. If legends are made about an item, legend lore will track such relics to any plane. Items can talk to rodents and avians within one mile of their lair ('regional effect', mentioned above). Power Plot-Point: Green dragons passionately give away such sentient treasure to favoured servants as well as servants that are valuable yet not trusted. An elf might live for half an eon with a treasured elven (common) magic item, unaware that it was briefly owned by a Green dragon and 'lost'. If the item is ambulatory ('able to move on its own') it would disappear a few times a month only to show up in the most odd locations ('how did it possibly end up here?' the owner would ask)
• Self-Help Tomes: In Dungeons & Dragons, self-help books actually work - for dragons triply so. Not only can they boost their incredibly high abilities but they can reuse these every century. They may prefer 'Clear Thought' but they would also value 'Leadership & Influence' books. Be sure to consider how long any dragon has owned such a book - each century gives them +2 to that specific ability until the game-cap of 30. After that, shared with some follower that accepts Reincarnation / is an elf / doesn't age.
• Amulet of Proof Against Detection / Location: These may well be given at birth by concerned Green parents. Dragon Hunters abound seeking wealth, power, reputation and parts for magic items. Other chromatic dragons are especially dangerous.
• Headband Of Intellect: Holding someone's intelligence hostage is extremely powerful, so this item makes sense if the Green dragon lends it out and maintains control over it. Hence, this is typically a sentient item (that gains 19 int, of course) and often the shape of a crown for a powerful kingdom. Bonus points for a matching set for all relevant royalty ('one for the king, the queen... and two chosen heir-regents)
• Staff o' Woody Lands: Allows one Awaken per day, no risk. So 333+ sentient servants per year, ten thousand every 300 years. A massive forest or a lot of long-lived birds like parrots, albatrosses & eagles. Bonus: this at-will Passing Without Traces makes for less ruined forests / your favourite stomping grounds is less stomped and not obviously your lair-territory.

There are many other magic items that offer perks, but few are as game-changing as the above listed. Having a Pearl of Power is useful if one of the Green dragon's spell slots is 3rd level or lower. Any active dragon needs someone they can trust with a Crystal Ball or scrying magic, typically just to survive.

Adventures For Green Dragons & Their Faithful Servants

If the collection of sentient creatures is a Green dragon's treasure hoard, their general quest lines become a bit predictable. Not only will they rarely see other dragons as a threat (except possibly those super-social Silvers), but they will be obsessed with the success of other mastermind-style bosses. Concern would go to the thinking-leaders: beholders, aboleths, illithid pacts, hag triads, disguised ogre-magi leaders, gray-drow princes, vampire overlords, morkoth mini-realms, questing / sentient Holy swords - basically anyone with a long-term agenda with a plan to make others take care of it. A red dragon taking over a mostly spent dwarven mine for a tonne of platinum ingots is of minimal interest compared to a Night hag trio-leader that has sold her ten thousandth soul this year.

• Lost Souls: An ancient Green made a number of powerful contracts that still stand and a balance is still owing. They need to track them down and re-negotiate owing debts. Some of these loyal followers are in petrification or under magical preservation (think Aurora-style preservation-sleep or such). Others have long lives and got interrupted by pressing matters. Some of these people the dragon owes, some of these people owe the dragon but circumstances changed to make the contract null-void. Players journey to these NPCs, resolve their specific crisis and then re-negotiate with them as proxy.
• A Deal You Cannot Refuse: A freshly minted Green dragon wyrmling needs to build up their follower-hoard! The wyrmling speaks through a familiar ('rodent or bird') and explains that he or she needs people to serve / has an entourage-retainer group of zero. This wee dragonling has access to some of his or her parent's hoard and is willing to share.
• Kobold Crisis: A hoard of kobolds had their High Lord, a black dragon, slain. As much as they claimed the loved him, they are very relieved it is dead. They seek to gain a new High Lord, another dragon! They clawed their way to the surface and are seeing sunlight for the first time. Would the PCs be so kind as to guide them to the nearest dragon of this forest? If not, perhaps their ghost would be more willing to help?
• Medusa Muse: A medusa has been gathering a solid menagerie of high quality statues and would like to sell them off (at no risk to herself). Could the group please get these hard-headed folks to the Great Green? She would love to have a contract carved in stone for future prospects as well.
• Unwanted War: Two kingdoms include many 'properties' (i.e. persons under oath, quest &/or contract) owned by one Great Green dragon. He or she wants them to last much longer! Both royal families are having a massive spat and are planning to go to war over it. Players: make it all nice, would you?
• DragonSlayer De-Silliness: A group of dragonslayers are seeking to kill the nasty (adult) Green dragon nearby. Truth is not this at all: the dragon has them on retainer and wants to recruit yet more dragon slayers to take out a much more powerful ancient Black that has become a deeply-green problem. By claiming they want to kill this Forest drake, they are not raising any suspicions with the Darker-dragon's network of spies / scrying / intelligencia. When the Green dragon raises enough dragon slayers they will be secretly re-purposed to take out this powerful black dragon. They will have to be tested on a series of treasure-gathering adventures first though...
• Stolen Maiden: A dragon was holding a maiden hostage, as dragons do. He was really enjoying her company, in fact, her hand was sworn to betrothal to a really horrible and powerful noble. She was actually enjoying herself whilst redecorating his lair, making it more comfortable for both of them - but that was not to last. Some assassin somehow stole her - and the drake wants her back! Players must find this nasty eloped noble and captured bride and kill off whomever deserves death.
• Forest's Revenge: Over three centuries a Green dragon has populated a forest with a hundred thousand sentient beings, mostly trees but some long-lived birds as well. Their dragon is dead. They want to come together and have her resurrected. Easy-peasy? Alas, a Night hag has got her soul - adventurers must rush before this DragonEssence is sold off to some Fiendish Circle.
• Deal Gone Bad: An elven king worked for a Green dragon for over a century to regain a person that he had 'on file'. When the elven king was done his extensive and elaborate quest... where was this person he was promised? Turns out that the Green dragon had thought that a century was plenty of time and the elven king finished up a couple of decades sooner than anticipated. The person the king wants is NOT 'on file' and the Green dragon has utterly no clue where she might be. Everyone is upset and the king is gathering forces to get revenge on this contract-breaking Green and take the hoard-wealth as collateral payment. The Green does not want to give up the money and really does not want to die. Could the players deal with the angry king (and his army), find-fetch the person and save the day?
• Kobold Gone Dragon: A small clash (clan? class? stash?) of half-dragon green kobold-urds have decided that they want to become dragon-newts. For that they need the matching draconic egg and fix up the cracks with gold leaf and enchant it so that they can be reborn as Green dragon wyrmlings (CR 2). This is actually a complex magical item to make and requires many parts, yet more (Green) dragon blood (and leftover parts like shed-claws and moulted scales). Also, this is a risky process - if adventurers could procure the help of a Diviner wizard that would be great? Note: not everyone is thrilled with the prospect of yet five more Green dragon wyrmlings - including the Green mom of the recently hatched wyrmlings that don't need the competition.
• Need Money, Please Send Green: A town languished under the rule of a Great Green dragon for many decades. So, they found a way to deal with him (or her) - petrification. Long story short: weaponized stone-ification worked and now the dragon is a statue in the town's square. Yay. Alas, as it turns out the dragon had been doing a great job. Now the town's prosperity is ruined. The nobles are asking for someone to find enough basilisk-bile and an alchemist to make that un-petrification elixir-juice-stuff. How hard could it be? Also problematic: the town's elven paladin round table is strongly against bringing the dragon back. They are above simply shattering the living statue... for now. But breathing life back into this Green dragon is verboten. Oh, the conflict! If you want time-pressure, put a distant army on its way to sack the lately-ill-equipped to defend itself as well. Players can struggle with invading orcs, capturing and mis-feeding a basilisk, a group of quasi-good-yet-bitter paladin elves and a half dozen powerful nobles all vying at greed (SOMEONE has to give the dragon its hoard back when it awakens... who took it?)