What is the highest armour class possible for an Ancient Green dragon?

Question

This posts asks what the highest armour class may be for humanoids. Similar to this question, I would like to know what the highest AC would be for a monster. Narrowing this down, let us look at improvement techniques possible for an Ancient Green dragon. As this is a BBED ('Big Bad Evil Dragon') they most likely have a slew of minions, guardians and servants - so please feel free to use any options that exist (in 5e) that may be spells, minions, a spare shield guardian, or anything else you like.

For stats on the Ancient Green dragon, has a base dexterity of 12 and a base AC of 21. I do not know if it could gain NPC levels of monk or barbarian, nor what use that would have. We can assume this dragon has the 'optional' spell casting abilities. According to old (possibly 3.5e) Forgotten Realms material these creatures are a minimum of 800 years old at 'ancient' and may live until past 1,200 years or more. They seem to have access to a lot of magic items. These things are gargantuan, so i have no idea, for example, what size shield they would need - or even if a shield works for them. I don't even know how big 'cover' would need to be in order to offer 3/4 protection. Would a Cloak of Protection even fit? I have specifically left this quite open as i would greatly value any and all possibilities that Stack Exchange has to offer.

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Question Recap: What is the best armour class possible for a 5e ancient Green dragon - assuming any & all options are available to it?

Answer 1

NPCs aren't limited like PCs are

Like Thomas Markov mentions in his answer, it is up to the DM to dictate monster statistics and you are perfectly within your right to edit the AC of your BBED to whatever figure makes the game enjoyable to you and your players.

What if I use RAW options?

If you were sticking to the options that are listed in the rulebooks, you can get a number of different answers depending on how your BBED wants to appear. To start with, the highest AC with a default ancient green dragon is 29. This is achieved by attuning to the following magic items (worn items resize to fit the wearer by default):

• staff of power
• two among ring of protection, cloak of protection, ioun stone: protection

... and the Blessing of Protection alternate reward. Then have minions cast warding bond and shield of faith on the dragon.

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If the doors are open to artifacts, you could get quite a bit higher at an AC of 33 with the same Blessing of Protection and spell buffs, along with these magic items:

• staff of power
• Orrery of the Wanderer (with the +1 AC random property in addition to the +1 granted by default)
• Wand of Orcus (with the +1 AC random property in addition to the +3 granted by default)

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If you chose to give the dragon class levels in accordance to the that option in the Dungeon Master's Guide, you could get as high as 50+ or 54+ with the artifacts. This is done with the following class distribution:

• Barbarian 1 --------------------- (for Unarmored Defense)
• Wizard (Bladesinging) 2 ------ (for Bladesong)
• Ranger (Hunter) 7 ------------- (for Multiattack Defense and the Martial Adept feat)
• Bard (College of Swords) 10 -- (for Defensive Flourish and Magical Secrets for haste)

This gives the following base AC calculation:

10 (base) +
10 (Dexterity after reading 9 manuals of quickness of action) +
10 (Constitution after reading 3 manuals of bodily health) +
10 (during Bladesong; Intelligence after reading 5 tomes of clear thought) + 2 (from the haste spell)

...with the potential for an additional 4 from Multiattack Defense, 1d6 from Evasive Footwork, and 1d10 from Defensive Flourish.

Then you just add the magic items and buffs from before to get the totals I mentioned.

Shield Spell

Another important utility for AC is access to the shield spell. If you are using classes, this is gained from being a wizard, but otherwise the dragon can get access once per day through the following variant in the Monster Manual:

VARIANT: DRAGONS AS INNATE SPELLCASTERS

Dragons are innately magical creatures that can master a few spells as they age, using this variant.

A young or older dragon can innately cast a number of spells equal to its Charisma modifier. Each spell can be cast once per day...

...or through a magic item (which would replace the least beneficial item from before):

• a ring of spell storing gives 5 castings before it needs to be refilled by some minions
• non-obsidian spell gems (which can be refilled) or spell scrolls (which are consumed) can each grant 1 additional casting if the dragon knows the spell from the variant rule
• staff of defense gives 10 charges where casting shield as an action (meaning it hampers the dragon's damage output when it casts the spell since it can only attack through Legendary Actions) costs 2, and it takes an average of 1.833 dawns to recover all the charges.

Haste Spell

The haste spell is another AC granting spell that a minion could cast (instead of the dragon itself like in the class levels case) on the dragon for +2 AC, but I would advise against it. If that minion ever loses concentration or dies, then the dragon would be affected by:

the target can't move or take actions until after its next turn, as a wave of lethargy sweeps over it

That being said, if this is a dynamic you would want to include in the fight, you can add 2 to the AC through this spell.

A Physical Shield

Another way to get a bit more out of the AC is to have the dragon equip a shield. This is a bit absurd in my opinion, but if you want to allow this, the dragon could get +5 AC from a +3 magic shield. The Basic Rules say after all:

When a nonhumanoid tries to wear an item, use your discretion as to whether the item functions as intended. A ring placed on a tentacle might work, but a creature with a snakelike tail instead of legs can't wear boots.

...so a DM could rule that a dragon can use a shield.

Answer 2

Frame challenge: As high as you want.

This question is very similar to your previous question: What can a dragon use to increase their armour class?

Rather than reinvent the wheel, I will echo my answer there, with some additional commentary:

If you’re the DM and want your dragon to be more of a challenge or have better survivability, modify its stats. Raise its AC or give it more HP.

There is no guidance found in any sourcebook about giving a dragon armor, but the Dungeon Master’s Guide encourages the DM to modify statblocks to suit their needs. So just do that. And if you want to make your dragon particularly cool or menacing, just say it has armor. You don’t need rules to do this, you’re not a player, you’re the DM. It’s your world, and if you want dragon armor in your world, then have dragon armor in your world.

You're the DM. The magic item rules in the Dungeon Master's Guide are player-centric, so using them on your NPCs is going to be up to you, they weren't designed with NPCs, especially non-humanoid NPCs, in mind.

As for spells, there are no spell lists given, so it is entirely up to you what spells you give the dragon. There are some spells which give temporary bonuses to armor class.

In the end, this question is focusing on the wrong thing. Based on the previous question I linked above, you seem to be a DM struggling to get out of the player's mindset. You don't have to think in terms of what mechanics are available to you in a strictly rules-as-written sense, because the rules for player character design were not written for NPC design. Build the big bad evil person that you want to build. You don't have to follow player character design principles.