Hi!

I started playing D&D a few years back and quickly discovered my love and aptitude for world creation. I’ve ran multiple campaigns and one-shots (fully custom and “canned” from a book) for players with a variety of experience levels, but have never ran a campaign for players higher than lvl 8 or 9. I want to get my feet wet in the higher level campaigns, but feel a bit daunted by all the player and monster capabilities. Any advice? I typically run games for 4 or 5 players. 

much appreciated!

You’ll want to find your Goldilocks zone on having enough monsters to challenge the party and balance out the action economy, but not so many monsters that it bogs you down. And make all of your encounters at least Hard to Deadly+ so you only need to run 3-4 encounters per adventuring day.

Thanks! Any tips on how many (if any) magical items to allow the group? I got in a situation once where the party was killing monsters way above their capabilities because they were all too decked out in magic equipment. 

The game was designed and balanced around the party not having many (if any) magic items. Other than an Uncommon weapon for each of the martial characters early in Tier-2 (5th-7th levels), and healing potions, that’s all the game was designed for. So that would be why you had that experience. It was also not designed to take into account things like feats of multiclassing either. Personally I give out a fair number of magic items because their cool, and then just up the difficulty to compensate. (Pretty much every encounter I run is Deadly+ In difficulty.)

Also, don’t fear really putting the hurt on the PCs, permanent character death is all but impossible after 3rdish level, so let the bodies hit the floor. Unless they really just don’t work as a team at all, statistically they will almost always survive unless you intentionally kill a PC. Even then, death is not the end if they have a good sized diamond and a cleric in the party.

And don’t worry so much about how they’re gonna prevail over the challenges you present them. They will almost always surprise you anyway. Just worry about challenging them adequately, let them figure out how to succeed. They’ll figure something out. (They always do.)

If you've already run a few campaigns to level 8 or 9 then you are doing well!

I've run a few high level games now (current one is level 15) and here are a few things I've picked up along the way with regards to monster and player capabilities:

• Have a rough plan for each session/arc as to how many encounters you will be having per short rest or per day.  You'll have to understand your own objectives about draining party resources in advance of the next challenge (be it combat, social, etc).
• Customize the monsters, feel free to add health, bonus actions, legendary actions etc to even out action economy. Dont add huge spell lists,  as you'll easily forget.  Just have some ideas about what they do and the spell DC.
• Let some fights have "phases" with different activation criteria to go to the next phase.
• The characters have an incredible amount of things they can do, specifically with divination and transportation spells.   As DM you have the information,  don't hoard it but let the characters use their abilities and spells to discover cool and interesting things in your world.
• Depending on your group -- let the players help shape the world.  As an example, if a character makes a history or religion roll and gets a nat 20 -- let them come up with what they found and describe it (ie Istus the god of fate has not been seen for 500 years).  Similarly,  let's say the party finds a magic key.  Consider letting them describe it and a few things about it (ie it opens a door that only opens during a full moon,  the key fades during the day, it opens something ancient, etc) -- then later you can use that to build an encounter/dungeon.
• Consider a tiered start.  Perhaps start at level 5, play for a session or two, then move to level 7, then to 9, then start the campaign proper.  This gives both you and the players some time to adjust.

Cheers, and hope that helps!

I'm currently running a campaign that has just recently reached level 18. It's a large group (8 people of which I have about 5 or 6 of them on regular basis) so the dmg encounter guidelines are only so helpful.

The first thing I would say is be wary of challenge calculators like in dndbeyond's encounter builder. Generally I've found my players can handle encounters a lot more easily than the numbers suggest (do take that with a grain of salt because my group is larger than your typical group and they do have quite a few useful and powerful magic items).

The second thing I would say is test your party - the challenge calculators aren't the most accurate so just throw things at your party to try and gauge what they can handle - for example my party has a Twilight Cleric so they can soak up a lot more damage than other parties of a similar level so I now through that bit more damage at them - it takes a bit of trial and error, but I would err on the start of going little lower at first. You can always make things a little bit harder in a fight (summon more monsters, give the boss a 2nd level like mythic or something) but it's a little harder to make things easier after the players have already seen what you're doing. Having said that, a recurring experience I've had is that I constantly underestimate my players capabilities so once you've tested them out a bit, don't be afraid to up the level of difficulties. 

Third, I've found using terrain and strategy rather than high level monsters is often the most challenging and fun for the players, like the assassin who move through walls and disappeared into shadows or the Kraken that swallowed their barbarian and tried to drag the rogue to his watery grave.

Fourth, I think someone else already mentioned this, but I edit monsters a lot or make my own. My players can each do about 50 damage on average per turn (and they've been able to do that since about lv 15 if I recall correctly). Very few high level monsters can stand up to that for more than a round 2 and they won't be able to do much damage to PCs before they go down => you need a monster with a lot of HP and a bunch of attacks OR several monsters with decent hp.

The two caveats I'd put on this is it does depend on the composition of the party and also my players do have a lot of magic items (I don't tend to hold back on this. Getting cool magic gear is a big part of my players fun - 18 levels, a bunch of awesome toys and they still haven't got tired of it)

However, making a fight challenging is roughly that simply. Make sure the enemies have the HP to suck up several blows and the ability to get off a lot of attacks (roughly the monsters should be taking an equivalent number of turns as your players, even if that's through legendary actions).

On General Storytelling:

I have less precise advice to offer here. Honestly, telling a story at lv 18 is like telling a story at lv 5. The only difference is the strength of the enemies and the difficulties of the encounters. So if what you've been doing has been working, keep doing that.

Lastly, I just want to say how much fun it has been playing at the higher levels: how far can I push my players? How do I make things challenging for them but not cause a TPK? How do I make interesting encounters for PCs who can roll like 26 on their primary skills easily (this one involves setting DCs a little higher than normal so they can't just get automatic successes)? It's all so much fun for me and them. I hope it will be for you too and I hope some of this helped. 

For martial characters,if you can manage level 8-9 you should be fine higher, mostly what they get from level is Moar Damage. High level casters are more difficult just because there's so many different options they might pull out (wizards have a particularly large and disruptive spell list).

There's a youtuber named Treantmonk who made a good video about this subject recently. You might benefit from seeing it if you want to search it out.

One thing to realize about high level dnd in 5e is that it is possible to break the game, and I think it's a good idea to address this early on with the players like session 0 if possible. It's easier to just make a social contract saying that they wont do stuff to intentionally break the game (like wanting to summon 100,000 Simulacrum) instead of trying to go through every possible scenario that might come up as DM and try to ban stuff that is game breaking.

Quote from Earl_of_Marmalade >>

Thanks! Any tips on how many (if any) magical items to allow the group? I got in a situation once where the party was killing monsters way above their capabilities because they were all too decked out in magic equipment. 

Magic items don't have to mean 'powerful combat items'

The party I DM for has a blast with all the weird magic items I give them -- stuff that's got some utility but is often just goofy or silly or strange. Their most prized possession is a rod of cake, a homebrew magic item I whipped up that turns things into cake (inspired by all the 'is it cake or not?' viral videos from a couple years ago). One of them has a robe of fuzzy friends, which is basically just a reskinned bag of tricks with a different list of possible animals. The rogue has lock picks made of finger bones that let her cast augury once a day if the questions relate to using the picks

You can reward the party without tipping the scales too much -- just get creative with the loot, rather than every item being combat oriented

Super helpful!

Awesome- thanks for the tip

Good idea- I’ve tried to do this recently, but of course everyone’s looking for that Vorpal sword drop lol

That’s what I’m nervous about- but I guess that’s half the fun? Maybe I’ll add in some magic resistant monsters so they need to use different tactics for different fights 

Quote from Earl_of_Marmalade >>

That’s what I’m nervous about- but I guess that’s half the fun? Maybe I’ll add in some magic resistant monsters so they need to use different tactics for different fights 

 The issue with high level casters isn't really damage, it's them entirely bypassing the fight.

The most important thing about magic items is to make sure the party is balanced against itself. It doesn't matter if they all have 2-3 magic items and are a group of level 12's punching as if they're level 15's - it only becomes a problem if they are all punching at different weights.

You can use the magic items as a rough concept of session 0, to get to know the players/characters before they even begin; if someone takes weapons and armour, then they are focussed on combat and would like to fight  things in the game. If they take utility items then they are more interested in making clever workarounds and exploring solutions which only come about in TTRPGs, with their infinite options that you don't get in video games.

And if they take the deck of many things, or that sort of thing, then expect chaos - throw your campaign plans out of the window, because they are picking tems which make the campaign!