Portals in dungeon. Following the correct order gets you to the center. Our heroes lead into trap/encounter rooms. Have it be the main puzzle

>dungeon is a bunker connected to excavation zone at the center of the crater by a rail system, with undead elephants for traction
>ballista turrets with skeleton crews along the rail lines
>bunker has a single heavily fortified gateway to the outside of the crater
>zombie pterodactyls patrol the crater
>the main facility is a decoy, the lich's lab is down another secret, underground stretch of rail

>>90211242
Take a page from Tomb of Annihilation and have a fake final boss.
Have numerous laboratories for the lich, where various decoys keep summoning trouble.
Scry and die combat.
Also look up 3.5e legendary dungeons and Overlord series as much as I hate the series.
Teleporter traps in almost everything
Split up the party and then send goons to kill them.
Hard part is to give them a way out if they're clever enough

>>90211242
> the entire place is filled with poisonous gas
> their is no safe floor to walk on. The lich can float at will. Imagine a room in the dungeon. The desks, bookshelves, and other furniture are all real, but the entire room has an illusory floor. The furniture are all magically floating in such a way to make them appear to be on the floor like normal. Under the illusion is something that the players fall into and die. If you want to be nice, make it something like acid or lava so the players can smell/hear it. But since it's a lich, I'd just fill the enter space under the illusion with waiting undead.
> The lich has actually been playing nice and become allies with some other group. If the players start declaring their intent to go after the lich, that organisation might have something to say about it.
> The ENTIRE PLACE is littered with undead and constructs. There is absolutely no way to sneak in unnoticed. Like there's a zombie for every 5ft square space inside.
> Because the lich can float at will, he doesn't need stairs or ladder. He just floats through tunnels and shutes.
> Every door and passage is covered by a simple illusion that the lich can see through. They're purely visual so the party can easily find them by touch, but they waste tonnes of time and behind some illusions are just monsters.
> Teleportation traps to completely random places. Walk into room. Oh, now you're in that town like 20 miles away. Walk into room. Oh, now you're in a cage in the liches study. The lich doesn't walk so he doesn't need floors that are safe to walk on.

Cursed water that blocks a path (jayquay).
Swimming through it makes you undead on failed save.
Telegraph danger: Swimming fish skeletons.
Cure: Remove curse or bring some of the water to a bishop level cleric to be sanctified and drank?

Phylactery hiding spot, in handle of powerful magic sword the players can loot in the dungeon.

>>90211242
You cant "always keep left" a labrynth with two or more floors
False doors can similarly hide the true path
If you're already dead (and have airtight doors) there's very little reason not to flood entire sections of the labrynth with toxic gas
An acid lake is also an excellent source of toxic gas (you can fly)
Pits arent just for jumping, they're also a good way to segregate a dungeon to keep otherwise hostile monsters in different areas
Let bandits base near your dungeon, they will steal an adventurer's supplies while the heros are dying in the dungeon. Without tents or rations or horses fleeing adventurers can be hunted down at your liesure
False lab rooms are great ways to trick adventurers into resting. Remember, an unequipped spellbook is a flammable spellbook. A damaged adventurer is not a rested adventurer. Bonus marks for multiple false walls that collapse into monster dens
There is nothing wrong with a passage only acessible through a water-trap or similar mechanism
Include vents and crawl spaces that overlook main passages. No harm in testing a cantrip or two before the big fight, and you can learn their resistances in advance

>>90211439
Combining these ideas:
Outside the dungeon the Lich pays a nearby bandit camp with ancient swords/weapons that are of no use to the Lich's studies. The bandits, in turn, hunt down scouts/adventurers that travel near the crater.

The crater is filled with a rail system, with various zombies/low ranking undead scouting the outer perimeter. There are multiple fake bunkers littered throughout the crater. The fake bunkers have undead created in them, so they're sort of spawning areas. Unless the party carefully observes the carts they won't be able to find the real dungeon

>>90211541
Fake bunkers are filled with teleport traps that send intruders to underground pits filled with various monsters. Successfully fighting their way out, the party would find themselves 2 towns away from the crater. Fake bunkers can also have spawning areas/undead pits so undead flow out of them.

Then moving into the real dungeon:
The rails deposit all the carts/valuables into a storage area where intelligent undead sort the artifacts from shiny garbage collected by the low-ranking undead. Intelligent undead chuck valuables into small portals that connect to artifact storage (more on this later), and useless objects into portals that connect to junk pits.

>>90211389
>>90211558
Various personnel portals lead into a labyrinth of portals, invisible doors, false ceilings, and false floors. False ceilings lead to more portals, and false floors lead to monster dens. A thick miasma covers this section of the dungeon, which can cause diseases and generally makes it hard to breathe.

>>90211558
Getting through the portals leads to undead/construct storage where undead are stored in bunks in the walls. Any noise wakes them, and creates a hellish hallway filled with countless undead.

Once through undead storage it leads to the main chambers
--Continued--

>>90211821
--Continued--
The main chambers are physically connected, but are disorientating as rooms are connected vertically and horizontally. Maybe include some non-euclidean architecture for proper fuckery. Everything is covered with illusions that the Lich can see through, but will leave adventurers walking into walls or traps.

The bottom room of the main chamber area contains an acid lake that has acid directly vacuumed up in pipes. The acid is then humidified into the air, creating an incredibly toxic mist that covers the main chamber, which deals serious damage to anything living. (Maybe use the acid mist as a way to find out which walls are fake, as mist seems to flow from illusion walls and ceilings)
(Also connect some of the acid to the artifact lab for acid testing)

>>90211596
Acid lake can fit this too. Although I doubt the party would be dumb enough to try and swim through it, an undead octopus type monster can try and grab anything nearby creatures to pull into the acid lake. If the party manages to kill it, they can harvest some of the acid for later use. (If the party ever wants to use Undead-turning acid)

>>90211648
Main chambers has 3 intelligent undead with the only purpose of testing adventurer's weakpoints using a vent system that does not connect to anything important. These 3 guards relay anything important to the Lich using message spells. So long as the adventurers are in the main chamber, these 3 intelligent undead will intentionally act like they're gone until the adventurers rest, and then pepper them with various spells to test their resistances, then run away to let the adventurers attack empty vents.
--Continued--

>>90211827
--Continued--
The final area:

At the top have a hallway that, once entered, teleports the party to an identical hallway. If the party notices, and tries to head back, it connects to the artifact testing area. If the party doesn't notice, and continues forward, they find a fake Lich on a throne, with a fake treasure room behind it. Commence fake boss battle. The fake treasure room is filled with countless cursed items, and a few valuables to fool the party.

If they try to leave the fake throne room it teleports them back to the entrance of the bunker, making them think they successfully beat the Lich.

If they found the artifact testing room, it'll connect to artifact storage, and areas that don't have acid mist (has to turn off acid mist in this area to ensure the artifacts aren't damaged) As there are smaller portals that connect the artifact storage to the entrance of the dungeon, this could make it so smarter players who transform their character into a valuable looking necklace or something might be teleported directly to the end of the dungeon. Could be a neat way of outsmarting the Lich?

This then connects to the Lich's chamber. Any ideas for the main bossfight? Given the whole place has a portal aesthetic I was thinking of a crazy mirror type area where the Lich teleports around through portal gates, and flings spells through the gates. Every turn the gates reconnect to different ones, creating a confusing fight where the wrong movement can lead to the weaker party members on the front line, and vice versa.

Either way, if the Lich thinks he's going to lose, he can open a portal gate in the ceiling that causes countless monsters to fall and create a monster den in the final chamber, using it as a distraction to try and teleport away.

What do you boys think? Is this enough? Feel free to continue adding your ideas, or criticize anything that sounds dumb. I have to go to bed, but I'm looking forward to your input, boys.

>>90211831
Sounds very nasty, remember to post the full dungeon map some time anon

>>90211242
signs giving directions, with undead hands pointing towards different parts of the dungeon. When an intruder is detected, the zombified hands rotate around, pointing in a different direction, throwing off intruders.

>>90211831
Given the portal theme, incorporating some environmental shenanigans would be dope.
>portal opens and water fills the room
obviously would be a last ditch thing because it'd impact whatever was lying around research wise
>portal opens, pouring negative energy through a planar gate into the room
would act like a healing mechanism for undead, at this point I'd assume most parties are warded against negative levels/attribute damage/negative energy, but now it puts a lot of pressure if the Lich gets a dispel off that hits those

>>90213616
The wilderness surrounding the crater is a dark forest with towering pines and evergreen trees. Guts, bones and broken weapons lie about the place.
A death knight and several wraith knights patrol this wilderness as a pack hunting down adventurers. Should the party enter the crater they are spotted by the zombie pteeodaactls which alert the death knights. Should the party try to escape the dungeon to long rest the death knights are waiting.

>>90211242
There is a single-floor maze carved through the living rock.

There is another, nearly identical, single-floor maze carved through the living rock directly below the first maze.

There is a poisonous fountain in the first maze.

There are some jewel-encrusted statues in the first maze, each bearing a different treasure in their cradling arms. Disturbing any of them triggers three spells:

>darkness (or cloudkill)
>mass sleep
>teleport

...they find themselves in the maze at the feet of the statue they had touched.

>they are actually at the same coordinates in the nearly identical second dungeon

The statues in this second maze are actually golems set to kill anyone who disturbs them or their treasures.

There is a fountain in the second maze like the one in the first, but it is not poisonous; any who drink it are teleported out of the second maze and into the first maze at the poisonous fountain.

As I said, the two mazes are identical at first... but the farther you go out, the more the second maze diverges from the first, first in subtle ways that might not be caught by an inattentive party, but gradually becoming more obvious, until the party finds a solid wall at what should be the entrance to a wide open hallway, or the maze suddenly emerges into a massive natural open cavern which just so happens to be the home of...
>roll on extremely dangerous monster table