I'm a new DM and I've created a magic item shop and some homebrew magic items for my players. I've heard plenty of horror stories about an excess of magic items, especially at early levels, ruining campaigns. I'm being cautious in what I give my players. However, if I do make a mistake and things go wrong, I'm wondering if there is a quick and dirty way to increase monster difficulty to account for these items. I could throw more monsters at players, but that risks dragging down combat and making it too long.

My thinking is increasing HP by 10%-20% and increasing monster attack rolls and save DCs by 1 or 2 is an easy way without much mental overhead.  

What are everyone's thoughts about increasing monster power this way?

Sure.  Anything is on the table.  Though generally most solutions are better than actually fudging rolls.  You want to avoid anything that the players could perceive as unfair or taking their agency away. 

Even the Monster Manual lists HP as a range for this reason.  They tell you the average HP, which is handy most of the time, but you aren't forced to use it.  You can use anything in that range and be strictly Rules As Written. But you're also free to just make up any monster stats you want. 

You can make 'elite' goblins, bring in reinforcements, give one of them a potion of dragons breath they suddenly use, have traps they spring in the middle of the battle, some magic spells, have one use the stats of a boss, etc.  Literally anything you can think of is possible.  Just try to make it feel natural. 

It's also totally okay if the PCs just steamroll a fight every now and then.  They feel good when they do it.  And you'll always have another fight ahead.  You can adjust the next one better from the start.  Good luck!

Ultimately combat difficulty boils down to two variables: how long the combat takes and how fast the combat drains your resources - aka the "burn rate."

Upping HP, defenses, and control effects increases the length of combat. Upping hit chance and damage dealt increases the burn rate. Upping number of monsters increases both, generally weighted a bit more towards burn rate.

Personally I like increasing the number of monsters, but if you're trying to avoid any lengthening of combat and going for simplicity then I'd only increase damage. Fast, bloody battles tend to make an impact on players. When you're hit for half your HP, you get serious real quick. When I only have a few enemies, I often double their damage (or add 2-3 damage dice to spells and other area effects).

I don't like increasing attack rolls, DCs, or overall CR because it can be a bit discouraging. It sucks when you invest a lot into AC or saves and then run into monsters that can easily hit you regardless. Let them still be hit infrequently, but when they are hit make sure it hurts.

Here are a couple of ways to do this:

• Up the monsters HP a bit or give them a Potion of Healing that they drink when severely wounded.
• Play them more tactically. Not every monster is a stupid beast with no knowledge of tactics or survival, in fact, most creatures are the exact opposite: They know what to do in and outside of a battle in order to survive and thrive. Without even adjusting any statistics, playing a monster tactically can drastically alter how challenging an encounter is. (For tips on how to run monsters tactically, check out this blog.)

Good point on fudging rolls. Definitely agree with that. 

My first instinct was to increase HP. The downside to this is that it could make fights a slog. That's why I considered increasing proficiency bonus instead; fights shouldn't take longer if I do that. 

I appreciate the last tip. I'm looking for an easy way to make encounters more challenging without thinking much. But if I have a little time, I like giving them potions, traps, a random spell, etc. Something to make them more interesting and not just more powerful. 

Good suggestion. I like the idea of making things "serious, fast" at least for battles that are meant to be a challenge. And your comments regarding attack rolls and DCs also make sense. Do you have a rule of thumb for increasing damage? Double the STR/DEX modifier? Up the damage dice by one or two, etc?

Yup, big fan of The Monsters Know What They're Doing. I try and play the monsters tactically sound but in accordance with their intelligence/skills. But I could balance it out by making even "dumb" monsters (goblins, orcs, etc) smarter. That would certainly be more fun for me!

Quote from mbenoit116 >>

Good suggestion. I like the idea of making things "serious, fast" at least for battles that are meant to be a challenge. And your comments regarding attack rolls and DCs also make sense. Do you have a rule of thumb for increasing damage? Double the STR/DEX modifier? Up the damage dice by one or two, etc?

 If this was pointed at me, I do it roughly like:

• More monsters than the party - an extra die of damage (or +4)
• Equal monsters to party - two extra dice of damage per turn (so if they make two attacks, each one gets one) (or +8)
• Fewer monsters than the party - double the damage

There are some outliers - if the party faces two fire giants, 12d6+14 may be a bit much.

One thing to remember about healing potions is that they are not great in battle because they consume your entire action. If it's not healing more damage than what the party deals in a round it's just extending the battle for no reason. So give them a bonus action "potion chug" or something if you go that route.

Why does the number of monsters matter? I’m assuming if there are fewer monsters, they’re either more difficult, or they aren’t meant to be really challenging.