D&D 5E Is this encounter too difficult?

[ECMO3]

I will admit my limitations with higher level play in 5e. Does this encounter seem too difficult? Not hard enough? It's designed for a party of level 14-15, with a good chance they will have had a long rest before entering the portal to this lair. Pardon the wall of text.

The Master Plotter’s Lair

The Master Plotter had chosen an abandoned mine to be used as their base of operations. They closed up the access tunnel so no unwelcome surprises would just wander in unannounced. Years prior, the mine was flooded after an underground aquifer was tapped into.

There are three ways in which one can gain entry into the Master Plotter’s lair: the magical portal from the Barrow, the Fey Circle, or clearing the rocks and debris from the tunnel. Due to the progression of the adventure, the most likely of these to be used by the party is the portal from the Barrow. The other entries will be described in more detail below.

With the exception of the magical portal in area 1, the area in this lair is dimly lit with heavy shadows all around. Area 1 glows brightly in red light. The pit itself descends into the darkness beyond most vision, finally ending in a pool of water 200 feet below. The water itself is only 20 feet deep, and is fed by underground passageways from a much larger aquifer. The water can mitigate falling damage, but only slightly. Any creature falling from the top level of the pit (areas 1 through 3) will suffer 15d6 points of bludgeoning damage, and any creature falling from the lower level (areas 4 and 5) will suffer 10d6 points of bludgeoning damage.

The walls of the pit are rough-hewn and can be climbed by succeeding on a DC 10 Athletics check each turn. Failure means the creature falls. The upper third of the pit, including the numbered areas, has walls covered with carved eyes of varying sizes and shapes. Many of these stone eyes will blink and follow the moving creatures they detect.


Master Plotter Lair Actions

The Master Plotter gains certain lair actions while they are not incapacitated and while they remain in their lair. The lair actions can occur anywhere within the lair. On initiative count 20 (losing initiative ties), The Master Plotter takes a lair action to cause one of the following effects. They can't use the same effect two rounds in a row:

• One of the eyes along the walls or ceiling emits a cone of dim purple light in a 60 ft. cone. This cone reveals any hidden or invisible creature within the area of effect.
• Any eye detaches from the wall. Use the Eyeling stat for this creature. Up to six eyelings can be active at any time.
• A 5 ft. section of the bridge or wooden planks breaks free.
• A 5 ft. radius area is overcome by violent shaking. Any creature within this area of effect must succeed on a DC 17 Dexterity saving throw or be shifted 5ft in a random direction and fall prone.
• A gust of wind rushes upward out of the darkness, extinguishing any non-magical flame and dispersing any vapor, cloud, or smoke in the lair.

The Master Plotter Tactics

Unless the party has been exceptionally stealthy, chances are that the Master Plotter is aware of them, either from telepathic communications with Emile, or from the eyes in the barrow. If they have had at least five minutes to prepare, they will be accompanied by six eyelings and will have any magical precautions and defenses up, preferring to stay hidden in the shadows at area 3, where they will wait for the party to enter the fragile wooden pathway before launching an attack. This attack will be with the goal of sending the PCs to their deaths deep within the pit.

Once combat is fully engaged, they will try to remain as mobile as possible, using hit and run tactics on the party. If reduced to zero or fewer hit points, or if pushed into the pit where they fall into darkness, they will undergo a transformation and shift to their second form (a large sized singular floating eye).

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Assuming at least 4 party members and at least one wizard and one other full caster - then no it is not too difficult.

 

[ad_hoc]

Also keep this in mind from pg. 15 of the PHB:

"In the third tier (levels 11-16)...these mighty adventurers often confront threats to whole regions and continents"

2 levels later they're facing threats to entire planes or even threats to the multiverse itself.

I think there is a mismatch of genres here. I don't know what plots the Master Plotter is up to but it doesn't read to me as a being who is going to control the fate of an entire continent.

CR aside I would expect the feel of this encounter to be late Tier 2.

Spoiler for RotFM:

Around 8th-9th level the PCs can take on and defeat a (weakened) diety who is a threat to Icewind Dale. That isn't even the climax of the adventure. It ends around the beginning of tier 3 when they explore the remains of a city from a civilization that had magic that rivalled the gods and are able to do things like travel through time and summon the Tarrasque.