So my campaign is drawing to an end, the party are in a race against time to retrieve an artefact...but first they need to make their way through a difficult network of caves...plan for there to be 3 layers, the last lair is a Dwarvan Forge deep within a volcano...

Its a party of 3 all Lvl 11...I was a little too generous with what magic items I allowed them purchase and acquire, so they are over powered, they mashed a legendary green dragon twice already(DOIP and the 3 follow ups)

First section is the following..

I was thinking of using the following...

Umber Hulks

Neogi

Molten Magma Roper 

Throw in a Hydra or two. I feel like people often forget about Hydras.

In the water at the exit? They have already fought a hydra 

So, I'd suggest looking at it from a different perspective: What challenge do you want to give the party?

This sounds odd, but let me give a low level example - your party have gotten used to the rank and file goblins who are for the most part pretty stupid. So you want to shake it up and challenge the party - give them a canyon environment that forces them through the valley. Up high on those canyon walls have ranged goblins.


You say you've allowed them to buy a lot of magic items - great! Have one of the rooms bathed in an Anti-Magic Field, this field is emanating from several pillars inside the room. Each pillar is guarded by a Behir. Give enough description that the pillars are the source of the anti-magic field thus allowing them to try and destroy the pillars at range. Their challenge then becomes balancing the destruction of the pillars in order to regain magic abilities, or luring the Behirs to a different location in order to kill them more easily. 

A single Purple worm can make for a really terrifying dungeon wide challenge. It can literally pop up anywhere and it can deal some real nasty effects.

A real powerful challenge that can have a similar effect to the Behir I mentioned earlier is the awesome Beholder. A lot of people forget just how nasty these things are. Their central eye casts a cone of anti-magic in a 150ft cone and each of their other eyes deals some truly terrifying damage potentials. The anti-magic cone will nullify the magic items really easily and yet still allow the players a freedom of choice in how they respond. What's really cool is that the central anti-magic eye can be focused on the spellcasters and the other eyes can cover its back, and sides meaning that the melee fighters are going to need to attack from the front to avoid the disintegration, or paralysis possibilities. 

You said it leads to a Dwarven Forge, so constructs might be a possibility. I love Iron Golem. Personally I'd use three of them against a group as powerful as you describe. 

A really powerful Fiend especially if surrounded by a gaggle of Imp is the Ultroloth. Their ability to just teleport away is really useful for providing a challenge. I tend to run the Ultroloth as an enemy that can make it's three melee and use either it's Teleport or Hypnotic Gaze, doing so makes it an infuriating and difficult enemy to take down. The imps make for a great swarm that can become invisible and if you make that a bonus action, they are really good to use to distract the party from the main enemy (the Ultroloth). This allows a granular control over the encounter balance too. You can effectively, if run well, challenge the party and make it feel to them like a punishing fight, but one that's still beatable eventually.

And turning to one of my hands down favourite enemies - an Ooze. The Elder Oblex is so much fun. I tend to slightly homebrew this one and remove the requirement for the Ooze to have stolen the memories of the simulacrum. So if the party split for whatever reason the Ooze creates a simulacrum of a party member in a different room. Or you can just have it be a lost child or something similar. It's a great choice to lure the party in and then hit them when they least expect.

For undead enemies, there is little more fearsome than a Mummy Lord and two Mummy. There's a better than even chance the party will make their saves but it can get scary when they fail their saves against the mummy lord. You need to have the mummy lord backed up by at least one if not two bog standard mummies though. They're way to easy to just obliterate otherwise. As an aside, this also works with a Vampire and a couple of Vampire Spawn too.

And finally, humanoid enemies - Drow. There are some really evil drow enemies (that I often reskin as a different race because why not). Of particular note is the Drow Inquisitor to whom you are unable to lie, who has a lance that can wick away your hit point maximum and on top of that can cast spells too. A couple of these, or even one of these supported by say a couple of Drow Shadowblade make for a good and fun fight.


Special mention as I remembered it at the end to the ever terrifying Mind Flayer Clairvoyant supported by five or six Intellect Devourer. People underestimate just how devastating an Intellect Devourer can be if you fail that Intelligence Saving throw. Literally last night I had a party member fall twice into Stunned by having their intelligence knocked to 0 by a failed save on their intelligence. Despite being level 12 his INT was only 10 (or a 0 modifier). I've had one party member in a different campaign outright die to one of these and that character was level 13 with an Intelligence of 9. However, what with their prevalence in Baldur's Gate 3, and the meme like nature of the Illithids at the moment I am tending to stay away from them.

like martin said, a lot of monsters can provide the same clannenge. you say its a dwarven mine? maybe somthing else moved in. something with too many eyes, and long tentacles that draw you toward its permeable mass, and then it spits out something else, something like you, but fundammentally diffrent. long, thin limbs, sharper teeth, garbeled speech that echoes for miles in this seemingly endless, yet claustorphobic tunnel network. maybe this thing moved in, and warped the dwarves, making them have long, stilt-like arms and legs and a walking gait almost like a gorrilla, and necks that reach down far enough to be at face level with the players, dispalying too-long teeth and eyes that still have the spark of intelligence, begging for the final release. you could have tentacles that try to grapple the players on the encounter table, and creepy, empty rooms, long, terrifying, unintelligable "speech" echoeing through the endless tunnels.

you dont need many monsters, just good atmosphere