5e Is this BBEG too strong for a party of 4 level 9s with tons of items?

[BerzerkerUnit]

For the plot of a 2-shot I'm running there are 4 Warlocks running amok. They're all turbo charged because they are somehow mainlining their Patron. The Fiendlock is possessed, the Feylock has stolen their patron's raiment, the Divinelock has their patron bound to them, and the Hexblade is technically like an AU version of its originator that became a kind of herald (full story below).

1st session was a murder mystery so they could get a little comfortable with their characters. The PCs are level 9. They randomly generated their class choice and randomly determined items from a curated list (this resulted in Deck of Many Things which was intentionally on the list because my Mystery sessions have been lead balloons and I felt giving the players a do chaos instead option was a good plan to keep the fun going).

This lead to the following:
Druid 7/Rogue 2- Deck of Illusions, Deck of Many Things, Necklace of Fireballs.

Monk 6/Sorcerer 9- Moonshot Bow (1/day makes a nat 1 a nat 20 and no range penalties out to a mile), Superior Potion of Healing, Iron Bands of Billaro (This PC got Donjon'd away by the Deck).

Artificer 7/Warlock 2 (I think, might be 8/1)- Staff of Frost, Wind Fan, Net Gun (Gun shoots nets 20/40, 2H, ammunition, loading), PC now 10th level due to Deck.

Monk 9- Steady Boots (reaction ignore forced movement, bonus action on a hit for 1d10 piercing), Love Potion, and Scorpion Gauntlets (Tavern Brawler feat but unarmed attack deals piercing, PB/day Giant Scorpion poison on a hit). Vorpal Sword added by Deck.

Wizard (Necro) 9- Items TBD, if the Wizard can solo his next monster encounter, he'll level to 10th. I've ruled creatures controlled by your magic and acting according to your will are an extension of the character.

Here's a link to the deadliest (https://drive.google.com/file/d/1lv2ET4AYpebHc9WcPI9XN8pl0uGcYmOC/view?usp=sharing) of the 4 warlocks.

Before engaging the PCs he'll use Mislead to converse with them, explaining a couple of things:
1. If the Druid agrees to come alone and be spirited away, he'll abandon this world, they won't have to fight him at all. (The Druid is an AU version of his originator's former ally and is bound for sad torment in his dread realm). Since it's a two-shot, the player may be more amenable to taking this offer.

2. If they kill him, he'll create a sphere that will begin a slow but inevitable process that will lead to the destruction of this world.

3. The Monk 9 has both the Rogue and Flames card from the deck, so if the PCs decide to engage anyway, the fiendlock and divinelock will intervene at a critical moment because they both want the pleasure of killing him.

So there's a lot of power on the PC end and a couple of panic buttons too. So is this just Pine boxes all around or do you think a group of level 9/10s have a shot?

[Yakk]

4 level 9 PCs sum to level 36.

16/36 is 0.44. Deadly starts at about 0.4.

If that CR is accurate, and they are optimized and have above standard gear, and the situation isn't too biased towards the bad guy, this should be a below deadly encounter.

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1. If the Druid agrees to come alone and be spirited away, he'll abandon this world, they won't have to fight him at all. (The Druid is an AU version of his originator's former ally and is bound for sad torment in his dread realm). Since it's a two-shot, the player may be more amenable to taking this offer.

2. If they kill him, he'll create a sphere that will begin a slow but inevitable process that will lead to the destruction of this world.
It looks like the fight is a trap. Winning is not killing him? Or is that a narrative element, not part of the difficulty.


3. The Monk 9 has both the Rogue and Flames card from the deck, so if the PCs decide to engage anyway, the fiendlock and divinelock will intervene at a critical moment because they both want the pleasure of killing him.
I don't understand this. Flames means the Monk has the enemy of a devil, and Rogue means the monk loses all possessions. What does that have to do with the fight?

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The warlock is a bit clunky as written.

Consuming Sphere at 1d12+21 doesn't average 30; it averages 28.

The reroll mechanic looks like something I'd cut. Those are fun on PCs, but the DM has more **** to worry about. "Force and Necrotic" -- in 5e you usually split it into different bundles.

Eldritch Smite: I'd honestly make this a crit mechanic. Less tracking, and mimics how a real warlock tends to use it.

Eldritch Breath: a bit of a trap honestly. Not better than a melee attack.

+7 to hit: That is abysmal for a CR 16. Up that to +10, or (more fun) add in an advantage mechanic.

Hexblade Curse: Fiddly, honestly. What I'd do is rewrite it to be thematically similar but more fun.

Have it grant advantage and crits on doubles on the d20s. This makes the above +7 to hit actually land, and makes the Smite "on a crit" mechanics go off a lot. I might also have the cursed foe lose max HP (!) from the warlock's necrotic damage, just because it makes PCs scared.

Blade of Disaster: The warlock has a +5 prof bonus, +5 charisma; should be +10 to hit.

Reactions: I'd change counterspell to a legendary action, and make it always require a check. Just because it is more fun (1d20+5 vs 10+spell level).

Hex Guard is ... boring. (and with counterspell as a reaction, it is dumb to use).

Reactions:
Counterblast: When a creature within 120' casts a spell, Niks can make an +10 ranged spell attack for 1d10+20 damage on the caster. If hit, caster must succeed on a concentration check against the damage, or the spell casting is disrupted.

Hex Guard: When hit by a melee attack, Niks can attack with either Consuming Sphere or Blade of Disaster (depending on if the Blade is within reach). If the counter attack hits, the triggering attack must reroll its attack roll.

Those are more fun reactions.

Legendary Action:
Void Recovery: A creature adjacent to Niks takes 1d6 necrotic damage, and Niks regains his reaction.
Eldritch Blast: You need to actually write the EB attack.
Relentless Hex: As written

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Shadow of Moil: Either roll that into stats, or remove
Summon Shadowspawn: Either toll that into stats, or remove
Blade of Disaster: Either roll that into stats, or remove
Maddening Darkness: Either roll that into stats, or remove

Planeshift: This reads as "get out of jail free" card, unless they counterspell.

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A problem with "here is a full spellcaster, can PCs survive" is that the tactics and how well you, as a DM, use them determine how nasty the creature really is.

I mean, the PCs are in a room that is difficult to get out of and maddening darkness lands, TPK. Repeat in a different scenario, no problem.

So I can't tell if CR 16 is reasonable or not, because the stats shown don't describe the difficulty of the encounter. For the most part, the encounter real challenge is the DM understanding the abilities of the Warlock, fixing the bugs in the presentation, and deciding how lethal the encounter will be.