What can a dragon use to increase their armour class? [closed]

Question

It is hard to survive as dragon - and yet they do. Despite being known for their nasty claws+bite, fear radiance and breath weapon - thus becoming a high priority target for nearly any ranged attack - they survive past eight centuries. How is this done?

Any long-lasting dragon would fathom bounded accuracy and 150'/600' range of bows. For example, a single-accidental pass of a small yet well-hidden community of wood elves ('longbow proficiency + high dexterity = +4 to hit') requires an AC of 25+, lest this wyrm suddenly become a pincushion.

How does a dragon increase their armour class? Possibilities abound: barding (need it be mithril for flight)? A shield (would it stack with their regular AC)?, retro-fitting magic plate armour (can a dragon wear dragon-scale armour? would-could-should they have a chest plate fitted)? What magic items fit 'gargantuan'? The list goes on / i have no idea where to begin... i just want a better armour class for a Great Wyrm.

I was hoping this would gain some kind of general answer rather than a slew of over-simplistic questions ('Could a dragon wear boiled leather barding - or must it be supple due to flexibility required for dragon-flight???' etc.)

Answer 1

Eternal vigilance is the price of liberty

The title of question asks for an AC improvement, but the OP says in comments "I had assumed that raising AC was the only solution. Please feel free to correct me." My answer will not touch on AC, but rather will focus on when and how a dragon moves, particularly Stealth and Perception.

The OP posits a "a single-accidental pass of a small yet well-hidden community of wood elves".

I take the 'accidental' and 'well-hidden' to mean that the dragon is flying over the elves without prior knowledge that they are there. Thus, the rules for Surprise are relevant.

From PHB: "Combat"

The GM determines who might be surprised. If neither side tries to be stealthy, they automatically notice each other. Otherwise, the GM compares the Dexterity (Stealth) checks of anyone Hiding with the passive Wisdom (Perception) score of each creature on the opposing side.

Any time the dragon needs to get somewhere, it should be choosing to fly at altitude - above darkvision range at night and, if possible, above bow range. Staying above where it can be targeted by missile weapons means its AC doesn't matter. If, for some reason, it is forced to fly near the ground, it should choose to move stealthily, repeatedly taking the Hide action any time it is within bowshot of the ground. Without stealth, the dragon and the elves will automatically notice each other, but if it is being stealthy, the dragon may escape notice. Even the youngest dragons have bonuses to Stealth and these improve as they age.

The wood-elf community, on the other hand, is stationary and its members are unlikely to be continually taking the Hide action 24/7 as they go about their lives - they would get nothing else done if they did so. Thus the dragon should likely notice the elves before they notice it and can then react accordingly.

If the dragon is flying at night when it hears the noise from the elven village and it is still outside of darkvision range, it could elect to simply continue on stealthily - with luck, its Stealth roll will beat the Passive Perception of every elf in the village and the active Perception rolls of their few sentinels, and it will be able to pass silently by. It should, of course, note the location of the village and remember not to fly accidentally above it again.

If the dragon is flying in the day (and the DM rules that Stealth is not possible while it is unobscured in plain sight) it will likely see the elves or their structures while still outside of bow range and can simply veer off and avoid the community altogether.

If for some reason (weather, terrain, darkness) the dragon is within bow range by the time it notices the community, it should immediately move outside of bow range (Dashing if that would take it out of normal range by the end of its move; if not, Dodging until it is out of normal range and then Dashing thereafter). Since the dragon was previously moving stealthily and at least some of the elves were not, it probably will be able to see them first, achieve surprise for many of them, and then extract itself from the situation before they can bring their numbers to bear.

A more challenging example would be a dedicated group of elven ambushers intentionally Hiding near where, for some reason, the dragon must pass at some point within bow range. Then, it is possible that the elves might be aware of the dragon's presence before it is of theirs and they might achieve surprise. Fortunately for the dragon, it has a good to great Perception and Passive Perception score and might have the Legendary Action Detect. Dragons should be difficult to surprise. The more ambushers there are, increasing the threat to the dragon, the more likely that at least one of them will get a Stealth score below the dragon's Passive Perception, and it will be warned.

Note 1: Some commenters have suggested that a larger number of ambushers would be more likely to surprise a dragon, when the DM permits a group check for Stealth, since the resulting value is more likely to be closer to the average. However, I believe that is a misapplication of the group check rules, which say (PHB 175) that they are "most useful when all the characters succeed or fail as a group." An ambush doesn't succeed or fail based on the whether all the elves are spotted or not - if the dragon spots any of the elves, then the ambush fails even if many or most of the elves are successfully hidden. Their success is thus determined by the single worst ambusher, not by the 'average' one and not by the total 'hiddenness'.]

Note 2: I do appreciate the time taken to do the combat calculations for ancient red dragons found in other answers. However, the OP specifically asked how dragons survive past eight centuries - so presumably they are interested in the younger age classes as well.

Answer 2

Frame challenge: Just raise its AC.

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.

Answer 3

The barding rules state that any armor can be barding, but they're unfortunately quite vague on which creatures are proficient with barding -- see Are mounts proficient in armour (barding)? for more on this topic. If your DM rules that dragons are proficient with armor, then a dragon can wear a +3 plate armor and have a 21 AC. The Adult Red Dragon has a 19 AC normally, so this helps a bit.

There are no rules that suggest that a monster can wear a shield ("shield-barding"?). We might imagine that a creature would need real arms to wear a shield, and not (eg) forelegs or wings. If your DM nonetheless rules that a dragon can wear a shield, you can factor that into the dragon's AC.

It's not clear whether dragons can wear armor while flying. The aaracokra can only wear light armor while flying; the winged tiefling can wear light or medium armor while flying. Your DM will have to issue a ruling on how much armor a dragon can wear while flying.

A ring of protection will give a dragon +1 AC.

Most dragons are not spellcasters, but a dragon might have an ally who is a spellcaster. The spell shield of faith grants +2 AC. The spell haste grants +2 AC as well as other benefits. The spell greater invisibility gives attackers disadvantage. If a dragon is a spellcaster, it can cast one of these spells on itself; dragons have pretty good Constitution saves, so it's unlikely to fail at concentration.

Most dragons have Frightful Presence, which can also give attackers disadvantage.

Answer 4

As has been pointed out, you're the GM so you get to say how this works. I'd probably just tweak the stats a big directly instead of trying to use items (because that approach lets you tweak the treasure independently of the stats, which is in and of itself important, just the raw materials in a full set of plate armor for a grown dragon is likely to be worth at least 10k gold even ignoring any value of the armor itself).

But AC is probably not the stat you should tweak here

AC effectively acts as a multiplier on the base HP values proportionate to how likely the attacker is to hit the target, because you need to make more attacks to deal the same amount of damage. The exact formula for this is 1 / chance to hit. Ignoring the possibility of advantage and disadvantage (which skew these numbers significantly), you get:

Minimum Roll to Hit	Effective Multiplier
20	20
19	10
18	6.666...
17	5
16	4
15	3.333...
14	2.875...
13	2.5
12	2.222...
11	2
10	1.818...
9	1.666...
8	1.538...
7	1.428...
6	1.333...
5	1.25
4	1.176...
3	1.111...
2	1.052...
1 (always hits)	1

It should be immediately obvious from this that increasing an already high AC has a much bigger effect on how many hits it takes to kill the target than increasing a low AC (and, conversely, increasing the chance to hit a target has diminishing returns). This asymmetry exists in any system that uses AC rules equivalent to 5e (including anything derived from the 3.5e or 5e SRD), as well as systems that use % damage reduction instead of AC (which includes a very large number of video games).

The net effect of this is that you have much more precise control of how 'tough' your creature is by tweaking HP instead of AC (especially for creatures with both stats high or both stats low).

Of course, there's also another factor to consider, most players can only get up to a +14 modifier on an attack without magical assistance (+6 prof with weapon at level 20, +5 on the modifier, and a +3 magic weapon, barbarians can instead get +16 on melee at level 20 due to the STR boost from their capstone). This means, based on the above table, that against fully kitted top level players, an ancient red dragon still has more than a 1.5x multiplier on it's health due to it's 22 AC, and therefore an effective HP of more than 816 if you're using average HP. That's an insane amount of HP to have, even with optimal weapons and average damage that's still at least a 20 round combat to kill it with a party of four.

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However...

You're making a lot of assumptions here about how 'at risk' the dragon would be. Looking at your original scenario, only about 15% of the shots made by the elves would actually hit an ancient red dragon, and if 100 of them make a single shot each, that's an average of about 67-68 damage they would deal. That's a non-negligible chunk of damage, but the ancient red dragon has a d20 hit die, and it has 28 of them, which means on average this is a drop in the bucket for it (in most cases it will take it at most one day to recover).

Even if we assume max damage on each shot, a group of elves with regular longbows would actually need 460 elves taking a single shot each to kill an average ancient red dragon. In reality, you actually need more than five thousand to ensure a kill if you are actually rolling for HP for the dragon instead of using the average (max HP is 812, so you need 5414 such elves to ensure a kill if you can assume that at least 15% will hit).