Anything BUT whatever dumb ass 💩 WotC has recently done or is currently doing, literally ANYTHING

Requirment: class that gets extra attack

Martials do not get extra attacks, instead, they get a pool of weapon damage dice = to the base weapon damage die + their level in martials classes (this stacks, so lvl 5 Barbarian, lvl 2 paladin has 7 dice). before they take the attack action, they divide their dice between a number of attacks (minimum 1 die per attack). they can also spend extra dice on their turn to attach rider effects similar to a battlemaster, just without the extra superiority die of damage. dual wielding adds the second base damage die to the dice pool, but you must split your dice between at least 2 attacks to gain this benefit. the martial rolls each attack roll, on a hit, the number of dice specified for that attack are rolled as the base weapon damage. magical weapons add extra damage die instead of +1/2/3 damage on a hit.

example: a level 20 champion fighter has the dueling fighting style. they wield a magical +3 longsword. they have 24d8 dice to spend on attacks (20 for lvl 20 fighter, 1 for longsword, 3 for magic sword), the fighter could choose to make one attack that deals 24d8+strength on a hit, six attacks that each deal 4d8+strength on a hit, or 24 attacks that deal 1d8+strength on a hit. or they could sacrifice up to 23 dice to add on rider effects (hoo boy, think about that, your a monster fighting this fighter and you get tripped, fientiend, shoved, disarmed, stunned, flat-footed, and like 20 other thinhgs all from one attack that dealt a paltry amount of damage

Entrance: the entrance to the dungeon is a stairway leading down to a landing and a mysterious stone door scribed with lettering in a language other than commo, but that the party is still likely to know (Elvish, Dwarvish, Orc, etc). the players must speak the password in the language to have the guard on the other side (a lazy goblin named Bubbo) open the door by a switch.

Room 1: The first room could be like a sort of tripwire trap, when activated arrows shoot out from holes in the walls. A DC 12 perception check to notice the tripwire, DC 14 sleight of hand to disarm it, and once the trap is disarmed they can cross safely.

Room 2: The second room is a test of charisma to teach them that fighting isn’t the only answer. A stone giant will be sitting in front of the door to the next room, and you need to persuade it to move. DC 12 persuasion, or if they make a good enough argument.

Room 3: The next room is a test of strength. 5 hobgoblins administer this room, and will only let the party pass if they can push a small crate filled with rocks from one side of the room to the other. The DC to move the crate is 15, Athletics. A mechanism is set so that when the box is put down at the destination, the door to the next room opens. instead of a DC 15 check, it requires a combined strength modifier of +4, so a normal martial has a low chance of doing it alone, so it teaches the value of teamwork, but the Martial can still contribute most of the work and feel like, as a standard party (fighter w/ +3 Str, rogue w/ +0 str, wizard w/ +0, cleric w/ str +1; approximately)

Room 4: The next room is a test of Intelligence. It’s a dark room, with the only light source coming from bioluminescent moss growing on the walls and floor. A brass cauldron sits in a wall alcove. There is a loft above the players when the enter the room, and seated on the loft is a goblin wizard named Bumbles that gives them instructions.

There are four types of moss — a red variety, a blue variety, a purple variety, and a green variety.

• The blue and green can be mixed to create a potion of healing.
• The blue and purple can be mixed to create a potion of flying.
• The green and purple can be mixed to create a philter of love.
• The red cannot be mixed with anything, and if it is the final brew will act as a potion of poison.
• Mixing three or more colors creates a diluted brew that tastes vaguely like basil and has no in-game effect.

The PCs are instructed to create a philter of love, which Bumbles needs to impress his crush. Figuring out which colors can be mixed with which requires a DC 12 Investigation check. Intelligence-based classes like wizards and artificers have advantage on the check. Four checks are theoretically needed to uncover all the possibilities, but the PCs need only to make as many checks necessary to discover the green+purple combination. They can also experiment by adding random moss to the cauldron and testing each potion to determine its effects.

The cauldron magically refills itself after each attempt, so previous experimentation won’t affect future results. Bumbles has no way to know which potions are which, so it’s entirely possible for the PCs to give him a non-love potion and move on regardless — the only downside being that the poor wizard’s date night is ruined.

Room 5: Room of Runes. This room appears to be a dead end. Each brick however has a rune etched on it. The runes are of an old, almost forgotten Magic tongue.

Arcana or History Check: DC 12. Just above average, especially for introduction dungeon.

On Success: The Runes speak of "Consuming Magic To See Beyond." A player should surmise that any magic spell can be cast in this room. When done so, the Runes ignite, revealing a door hidden by magic (and undetectable by other methods).

Room 6: The Mirror Room: Inside this bland room there is a single mirror. Upon approaching it, a shimmering image of a bard appears. "Good day, the name's Arius Songstrider. It would seem a hag has trapped me inside this mirror. Apparently she wasn't pleased with me making a song about how ugly she was. The hag believes that no one would care enough to try and free me from this prison." The bard shrugs. "Well, it's true," he says beneath his breath. "She was rather ugly. Now that said, it's highly recommended that you do not break the mirror. It's true what they say you know - about the bad luck. The bad luck comes from hags because they often use mirrors to travel between areas. And when they discover someone has broken one of their passage ways they will bestow a curse upon you."

The party can speak with Arius - and he will eventually reveal, the only way to set him free is for them to preform a song about him and his woes - to prove that someone cares enough to get him out of his prison.

Someone in the party (is there a bard present?) can do a Performance check - or the party as a whole can perform as a "band" - banging on their weapons, staves on the ground, to create musical "sounds." The DC is a merely a DC 12 Performance check - if it's done as a Group - just half or more of the party need to succeed.

Doing so will free Arius from the mirror - and the party can now have their first NPC.

Room 7:  The next room is a test of Constitution. A small pedestal will have a bottle with a drink in it. A man in a mask will be on a balcony. A lever is next to him, which is the trigger to open the door to the next room. Once the party enters, he will speak:

"So you seek to brave this perilous place? Try this elixir here!" He points to the potion. "Once someone drinks it and doesn't vomit, I'll let you pass."

Any character can drink the drink, forcing them to make a DC 10 constitution saving throw or gag and vomit uncontrollably for a small bit. Otherwise, no effect. Once a character has drinken once, they will always have the same reaction, forcing them to try different people instead of one person drinking it over and over.

Once a character succeeds:

The man claps. "Very good! Onward to the next room you go." He pulls the lever and the door opens. 

If the rare event nobody succeeds occurs: 

The man sighs. "None of you have a proper immune system. Kids these days, not gaining their vitamins." He pulls a different lever the party didn't see before, and a noxious gas fills the room, causing the party to fall asleep for around 10-30 minutes. When they wake up, the door to the next room is open and the man is gone.

Room 8: The Enchantment Room: As the party enters the room - the door behind them seals shut - and seemingly vanishes! (Any attempts to locate the door they'd just come through will result in no success). The room appears to be a large temple - empty pews, now rotting line up in seven rows. Ancient stained glass is cracked and broken - but the view outside seems to be a dark abyss. Down the long row of pews, a single statue with its arms outstretched gazes directly at the door all of you have come in.

As the party searches around the room and/or approaches the statue, the lone statue in the room suddenly animates. "Welcome. This room was once a temple to a deity, long since forgotten. Over the ages the connection to the deity has been severed - and as such - I have but one more blessing left in me. Who among you will step forward so that I might temporarily enhance your weapon?"

The statue will bless a weapon - temporarily making it a +1 to hit, +1 to damage.

The final room probably should be some form of combat, I assume.

After the party exits these ten rooms, the enhancement will cease to exist.

After the statue blesses someone's weapon - it will crumble in a burst of light - revealing a hole where it once stood that the party can descend down into the next room.

EDIT: You could even at this point use "Survival" checks to tie a secure knot on something before they descend, and then have people make Athletics check to descend down the rope. Falling would only be about 10 to 15 feet, so minimal damage if someone fell?

Room 9: The Big Boss Throwdown!

after descending to the boss room from room 8, the party lowes themself onto a small island in an underground lake where an Ogre is. the ogre got trapped down here through a tunnel that partially collapsed, but that can still fit medium creatures if they squeeze, (the tunnel leads to room 10, prolly a treasure/reward room). the Ogre is asleep, dozing on a throne (really giving the rogue a chance to shine), and the party must kill it. to make the Ogre not able to one hit kill the whole party, give it the following adjustments:

• The ogre has a strength of 16, as spending time trapped in the cave has made it weak. 
• The ogre wields a normal club rather than a greatclub, as it once had a greatclub that broke when it tried to smash its way out of the cavern, so now its a smaller club, the party could possibly parley with the beast by offering to fix the club (mending, anybody?), which the ogre cuddles like a stuffed animal while sleeping

This makes the Ogre has +5 to hit, on a hit it deals 8 (2d4 + 3) Bludgeoning damage.

Room 10: The Charmer: In Room 10, there is a statue of an old woman - whom Arius Songstrider (from Room 6) - will say it's the hag who trapped him - and that she seems to have been turned to stone. The statue of the Hag is indeed the very same hag - it would seem that in order to preserve her own life - she embraced the stone of this dungeon. Now seeing that the people she trapped in here (the party) have also managed to free Arius - she will attempt to charm them.

Each party member where hear something in their head that ties to their background - and sounds peaceful and beautiful - which is the Hag trying to convince the party to remain trapped in the dungeon (she will feed off their life force). The party must make a Wisdom Save DC 12 (just above average) to break/shake off the effect of the Hag's voice. Success means they're immune from this hag's charm for up to 24 hours - failure means the character is considered Stunned (A stunned creature is incapacitated, can't move, and can speak only falteringly. The creature automatically fails Strength and Dexterity saving throws. Attack rolls against the creature have advantage.) The save may be repeated each round at the end of their turn.

Once the stone hag is reduced to half her hit points, she will spring a trap - that drops the floor beneath the party. The party will need to make a Dexterity Save (DC 10) to avoid the trap. Falling into the trap will do 1d6 damage - and the party will land in an unusual bed of flowers. Upon contact the flowers emit pollen that will render the party unconscious (DC 10 Constitution save). Climbing out of the pit will require an Athletics check DC 11. The pit is non lethal because the hag needs to feed off the life essence of the party.

Note: Whoever got their weapon Enchanted  - see Room 8 - will do double damage on every hit used with that weapon against the hag. The Ancient Temple (Room 8) is the only room the Hag has never been able to enter or corrupt.