Hiiii.  So, I run a group and play in a different group.  Occasionally the DM of the group I play in likes me to run one shots to practice my DMing skills.  However, this time around I won't be using some pre-written from a book.  Through Instagram, I found an account called Plus Five Charisma that posted an interesting layout.  A five room dungeon with the following set up:

Room 1, the entrance, is trapped.  Room 2 contains a puzzle.  Room three contains a fork in the road with one of the forks leading to red herring (not sure EXACTLY what this means so I just set it so there are two doors, one is fake and reveals a part of the wall, the other contains the path to the next room...that can be adjusted based on what advice anyone has to give).  Room 4 is the boss fight and Room 5 is the treasure room containing a surprise encounter before the party can retrieve the treasure.

I'm going to be building the dungeon from scratching choosing the trap, puzzle and both encounters.  But since I'm on my own (usually when setting up non pre-written stuff for the group I run, the DM of the group I play in assists me and helps with balancing, but has asked to be left out this time so he can be fully surprised when I run this small-scale dungeon), I need help to get an idea of balance and level recommendation.

Now, I have a couple of questions the first being what level should I have the players set their characters to.  I DO have an anchoring point for anyone who wants to give me some advice to work with: The trap in the entrance room is called The Lightning Rod and comes from a 3rd party book I found at a bookstore.  The trap, when triggered, requires the person who activated and any creatures within five feet of that character it to succeed on a DC 17 Dex save or take 10d10 lightning damage.

If three or more fail the save, then the player who triggered the trap must succeed on a DC 17 Con saving through or take 8d10 lighting + 8d10 necrotic damage.  If the player's HP hits zero they're reduced to a pile of ash.  Now to me I feel like that's about Level 15 or 16, but I might have to set the player level higher OR lower.  The book doesn't exactly share challenge ratings, but the runes that activate the trap can be dispelled with Dispel Magic (which is a 7th level spell).

So my first question is, with this trap in play, what should I tell the group to set their one shot character levels to?  My second question is what would be an appropriate encounter for the boss in Room 4 (recommending monsters or just telling me some appropriate challenge ratings will suffice, and if it's the latter, I'll look into some encounters)?  The third and final question is, should the surprise encounter in the final room be easier or the same level as the boss (I feel it should be easier, but I still don't know what would be TOO weak of an encounter/CR level).

Answering these questions will help me balance the dungeon and give the group a fair chance to complete it AND gives me material to study and practice with.  For example telling me which CR levels are best for which player character levels, will give me the opportunity to learn what's too overpowered and too weak and allow me to craft quests and such on my own.  If I wasn't running this for the group I play in, I'd ask my DM for advice...but since I'm DMing for HIM this time, as I said, he wants to be surprised.

Oh I forgot to add: It will be a party of three players.

My advice, if you're still new enough to need assistance in balancing encounters: don't go above level 10. Honestly, don't go above level 5.

But if you're set on including that trap, then remember that if a monster's CR is equal to the average level of a party of 4, then it's roughly a balanced encounter (might be an easy encounter for experiences players, but it shouldn't be a cakewalk). What I mean is that you should choose a creature whose CR equals the level of the party, provided you have a group of 4 players. With more players the CR should be slightly higher than their character's levels, and the opposite for if there are less than 4 players. 

Quote from VanZoeren >>

My advice, if you're still new enough to need assistance in balancing encounters: don't go above level 10. Honestly, don't go above level 5.

But if you're set on including that trap, then remember that if a monster's CR is equal to the average level of a party of 4, then it's roughly a balanced encounter (might be an easy encounter for experiences players, but it shouldn't be a cakewalk). What I mean is that you should choose a creature whose CR equals the level of the party, provided you have a group of 4 players. With more players the CR should be slightly higher than their character's levels, and the opposite for if there are less than 4 players. 

Well, I can remove the trap if need be.  There are standard set numbers (55 for the initial Dex failure and a 44/44 split on the follow up roll IF the follow up is necessary) so I can lower the danger threshold.  However, to make sure I understand this, I believe then if a party of three players are all at Level 10, then a monster's  CR should be 9 or lower, yes?

Correct. But just know that that's usually considered a "Medium" encounter, which won't necessarily be super difficult for players of that level. If you have multiple combats you can gauge it better -  but for a one shot the balancing might be tougher, since you won't know exactly the strength of your party. 

D10 average is 5.5, so average damage from that trap is going to be 55 points since the DC is high.

treat the trap like a monster. 55 to 100 points of damage in most cases will kill most characters of less than 12th level, and only the big hit die folks will survive it.

if they have a d6 hit die, the average is 3.5. They need to have at least 16 hit dice to have roughly a 50% chance of survival.

so, your trap is roughly a level 17 or higher just on damage. After some typical math for improvements made that might reduce damage by half, you are still talking at least 13th level.

A lot will depend on how it is triggered, how big the room is (unless it is 15 by 15 at least, the whole party is likely to be hit), that kind of thing.

so my first thing is “what level do you want them to be?”.  That will guide your damage for traps. If you want a total party kill, keep the trap as is, send them in at 9th level. A trap to start that big also says “big treasure”, and this is D&D so you get what you earn. Reward should be worth it.

the CR guidance is good — for a party of three, step down one CR for a single critter, or 2 if you add several minions (3 at least, 4 if you want to make ‘em sweat /bleed ).

remember to use all six senses in your descriptions of everything — the sixth being the vibe of the place, the feel of it.

cackle randomly after a player says something. Other times, listen and nod to what they say with a blank expression, roll a d20, make a note, keep nodding.

If it was me, I would say 7th level, drop save DC to 12, damage to 4d10 on fail, 2d10 on success. Use a CR 6 baddie, four CR 1/2 minions. As the baddie dies, it should warn the players that served a master who will now come for them.

For red herring (a fake out) I would have an idol with ruby eyes — except the gems are really a slow acting contact poison that puts people to sleep.

for surprise guardian, a homonculous or similar familiar like critter, tasked with defending the hoard, who at first will thank them for stopping the boss, and will offer them literal wooden coins in gratitude. When they take treasure, it attacks.

when they kill it, they hear a terrifying sound (rolls against wisdom, CR 12 or 15 or be scared witless, including voiding self) and the whole place begins to shake, with a strong feeling that something really terrible is coming, something that could end them in a heartbeat.

it is all illusion, of course. But who is going to cast dispel magic after the critter is dead?

in among the treasure, though, is a silver necklace from which hangs a single claw, heavy, the cut end set with silver. Not worth much, not magical under examination, but if anyone succeeds on checking it is a dragon claw that was once said to be used by an old wizard obsessed with extending his lifespan…

But that’s me off the cuff. 

and now there is a story hook for a later adventure involving a lich who is upset his favorite hoard and most precious possession has been stolen by grave robbers….