One of the Fighters in my party has an AC of 21 at level 5. He wears chainmail which gives an AC of 16, has the Defense Fighting Style which gives a +1 to AC when wearing armor, and a +2 shield (found in an ancient cave) which adds an additional +4 to his AC. As the DM, I am having an issue with any number of creature hitting him. I understand that I allowed this to happen, but I'm starting to regret it. How can I at least give this Fighter an actual fight without being unfair?
Hit your fighter in the NADs1
AC isn't the only way to cause damage to a fighter.
- Utilize creatures that force saving throws. High AC won't save you from a fireball
- Give them hard choices. AC won't save you when you have to choose between helping the rogue or the wizard…and you only have time to help one
- Talk to your players. Let them know the high AC is making things difficult for you, and you'd like to ratchet it back some. See if they'd be willing to trade in the +2 shield for an alternative magic shield with a property instead of AC bonus
The biggest thing to keep in mind is that AC isn't the end-all-be-all in D&D. Yes, it's important, but there are avenues around it.
1NADs = Non-AC Defenses. In D&D 4e you didn't have saving throws, rather you had static defense values: AC, Fortitude (Str/Con), Reflex (Dex/Int), and Will (Wis/Cha). A common reply to fighters with super-high AC was "hit 'em in the NADs!", meaning that a high AC usually meant a low Reflex and/or Will defense—fighters of course having a high Str or Con meant a good Fortitude defense.
Design encounters to target a wider variety of defenses
Bounded accuracy in 5e means that previously small bonuses (like +2 AC from a magic shield) have a very large impact. If they are on already top-of-the-line items for some type of optimization (such as full plate are shield are for AC) this can become game breaking. From my experience DMing for a couple of PCs who've chosen the very tanky path the best way to keep the challenge up is to constantly mix in enemies targeting something other than AC.
Luckily you've got lots of options. Monsters that target AC can be easily converted to target a save without affecting their challenge rating by using table on DMG p.274. Simply look up their current attack bonus and substitute it for a save DC in the same row against an appropriate stat. Next re-fluff the attack so it makes sense story wise that the PC is working to resist something. Some examples:
- An adult brass dragon's tail attack is normally +12 to hit. Instead ours will be wicked slap of the tail meant to send the much smaller PC flying. Save DC 21 dex to dodge out of the way or take the listed damage and be knocked back.
- A nightmare's hooves are normally +6 to hit. Instead ours will be the soul crushing terror of the devil horse looming over you taxes your mind. DC15 intelligence save to keep your wits about you or take the listed damage (now psychic instead of bludgeoning).
- A bone naga's bite is normally +5 to hit. Instead ours has a strange soothing pattern to its motions and only the most perceptive can recognize it is about to strike. DC14 wisdom save or take the damage.
This type of design also makes it very easy to provide effective on the fly results for improvised action. Just figure out what sort of save the victim should be trying to make and use roughly the attackers normal damage. I've also tried mixing in skill contests where the monster is considered proficient in whatever skill they are trying to attack with. This has a nice effect where PC skills matter more in combat and players, now reminded those skills are available, look for more chances to use them.
A word of warning this should not be done one every single monster in every single encounter. AC should still have its time to shine so leave smaller things like goblin crossbows and orc fists as simply targeting it. This lets that awesome magical shield the PCs found keep feeling awesome as it lets them shrug off all but the luckiest of the nuisance creatures.
Also remember turn about is fair play! PCs improvising actions should be given the same (and often better to encourage the practice) chances to target unusual saves in exchange for coming up with interesting interpretations of their attacks. In my games this has led to all around much more interesting combat.
Seriously, you shouldn't "deal" with his armor class. Like, at all. As pointed by CTWind, "his AC is obtainable via completely mundane means". He got it. He could've get it anyway.
But even if he would get higher AC than normally possible, let him. Do not make the impression of "We don't need to care or prepare, if we'll find something good DM will nerf it anyway". That's very bad for morale. Making challenges challenging is all good and funny, removing hard/luckily earned advantage can ruin game for many players.
A good campaign will have many different challenges. Traps, magic, diplomatic issues. OK, one kind of challenges is now a bit less challenging for one player. That's not a problem. It'll even out in a level or two. Other characters will seek to get their defenses boosted, he'll start encountering tougher monsters. Big Bad Evil Guys will learn that their opponent is "this guy wielding legendary shield" and plan accordingly. Ultimately, +2 on 20 roll is just a 10% improvement / penalty. If there was ten guys with bows, now you need 11-12 to get roughly the same effect. Shouldn't be a problem, if you explain it in game and it is NPC sergeant who dealt with the problem, not you directly.
In the meantime, it is his reward. Let him enjoy it. If he became unstoppable for goblins, so be it, smile and make it a goblin legend! Just don't forget other players wants to be legends in their fields of expertise, too.
I have a similar situation, but also 30+ years experience with D&D, so let me pile on:
- Hit him in the NADs (as described above)... Saving Throws, Saving Throws, Saving Throws.
- Non-attack type combat actions. These are my favorite. Wearing heavy plate armor and a stack of enchantments? TACKLE!! GRAPPLE!! TRIP!! How about a net? Ever see a knight in full plate try to get up off his back?
- Environmental Hazards. Make it slippery. Make it freezing. Make it on fire. Make it sticky. He's HEAVY... make the ledge crumble from underneath him. Make him cross the rotting, rickety rope bridge over the chasm of doom.
- Drown him. Make him rue the day he was too arrogant to take OFF his heavy metal armor before he got on the boat. Mine (a paladin) went to the bottom of the ocean when he was washed overboard by a rogue wave during a raging storm.
- Use an exotic monster with an exotic effect. The mudtiger from the old "A0 Danger at Darkshelf Quarry" module was hilarious... an area-effect electrical shock like an electric eel, but it also randomly (and constantly changing) magnetizes metal. His metal armor, shield and weapons attract one second, repel the next... causing him disadvantage on attacks/saves/skills involving any of the magnetized metal.
You and I wouldn't fight fair if we were faced with a guy in metal armor and razor-sharp weaponry. Your adversaries shouldn't either (assuming they are intelligent). But make sure it fits in the storyline why these High-AC Countermeasures are here. And again, don't use these techniques in EVERY engagement against him... let his high AC work for him most of the time. Use these ideas every once in a while to keep him challenged.
Happy Adventuring! -- DM Mike
Attack the rest of the party
The fighter has chosen to be hard to hit, and in doing so chosen not to be as damaging as, say a fighter with a two-handed weapon and style. Monsters are less penalised leaving his reach, so can go past him and attack the squishy back row characters.
But what if the party utilises tactics and positioning to prevent that?
Let him shine at doing what he chose to be good at
So the party uses a doorway to stop monsters getting around the fighter - they wail ineffectually at his AC21. This is a good thing, presumably the sort of situation that the player wanted to get into.
Mix things up (but not too much)
The other answers do a good job of suggesting ways of threatening the fighter personally, but I recommend only doing this sparingly if the players are enjoying having a meat shield
Chainmail? Oh I love heavy metal!
2nd-level transmutation/Casting Time: 1 action/Range: 60 feet/Components: V, S, M (a piece of iron and a flame)/Duration: Concentration, up to 1 minute
Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.
Many of the other answers address how to handle this through alternate combat so I'll focus on non-combat options for your situation.
Give the shield a yet undiscovered downside.
its cursed and it attracts undead
when it blocks a natural 19 from an opponent it lets out a loud ear splitting ringing which can be heard for a long distance
in the presence of goblins it shines like daylight
Being a +2 shield it must have some story behind it. Use it as a Macguffin.
It is famous and draws attention (both good and bad) wherever they go.
It is infused with the essence of some ancient bad guy. It needs to be destroyed in a volcano (insert your own trope here).
Other people have been questing for it and don't believe the character is worthy to have the item and demand it be surrendered.
Give the player a reason to swap the shield for something else
Returning it to the local king grants the player a small keep/plot of land/noble title/etc. (Also fits in the Macguffin category.)
If he finds a good magical two-handed weapon (which no-one else in the party can wield) then he can give the shield to someone else in the party to use.
Rust Monsters in 5e are CR 1/2 with an attack that targets worn armors/shields and requires a DEX saving throw. Each failure causes a permanent -1 AC, until eventually the armor/shield is destroyed. This would not work on that specific +2 shield (as it is magical and thus immune to this monstrous ability), but it would force the fighter to either return to town or continue a dungeon with nerfed chainmail. And if the chainmail is destroyed, the Defense Fighting Style would be suspended because it requires worn armor.
Just pretend that you rolled for them "randomly" on your wandering monster table.