D&D 5E Help with 19th level boss fight

Wow, lots of great responses here! While I absorb them, here is some extra detail as requested by posters.

Statblocks for the Tl'a'ikith and the Kr'y'izoth

Party Equipment
My vague wording in the OP has caused some confusion on this front. When I observed that they had no magic weapons, what I meant was that inside the anti-magic field they'll be lacking them, which is vitally important against a Lich who is immune to normal ones. Here is a list of magic items possessed by the party. They also have massive piles of consumables, for example a giant-sized-potion of diminution, courtesy of Lyn Armaal.

Dwarf Monk - Bronze Horn of Valhalla, Ring of Three Wishes, Sun Blade, Bracers of Defense, Headband of Intellect, Ioun Stone of Insight, Ring of Swimming. Most importantly, he can Dodge with AC 21 and still attack multiple times a round. As a result, he isn't hit very often - my dice love to miss him. We use the fan-made Way of Four Elements for him, so he's often in Investiture of Ice form.

Half Elf Arcane Trickster - Shortbow +1, Shortsword +2, Amulet against Proof and Location, Ring of Mind Shielding, Cloak of Invisibility, Broom of Flying, Bag of Holding with about twenty spell scrolls in it, plus a Banner of the Krig Rune which they've never used. Normally flies around on the Broom of Flying with the Cloak of Invisibility active, and is immune to the Truesight spell since that's divination and blocked by his other items. In normal circumstances, he's functionally unkillable by enemies without native truesight.

Dwarf Cleric of Thor - Belt of Hill Giant Strength, Rod of Lordly Might, Cape of the Mountebank, Ioun Stone of Regeneration, Boots of Springing and Leaping, Figurine of Wondrous Power (never used). The party's main source of AoE damage since the Sorcerer's player changed to the Champion. A surprisingly versatile character, who can be irritating to get into melee with due to various pushbacks.

Dragonborn Oath of Devotion Paladin - Vorpal Greatsword, Periapt of Wound Closure, Cloak of Protection, Necklace of Fireballs. Makes everyone within 30ft. immune to Charm spells and Fear, hits like a freight train.

Half-Orc Champion Fighter - Gem of Seeing, Longbow +2, Bracers of Archery. Even with the Sharpshooter feat banned, this character does a disgusting amount of damage every round, not least because the player crits at least once per turn. Weakest defences in the party at AC17, but has over 200 hp.

Don't let anyone tell you that the 5th edition treasure tables don't give out magic items - they absolutely do, just only after level 9. As you can see, the party have a crazy number of items, and across the board have good AC, magical damage, great movement options, and lots of utility. The last time they felt in real danger of a TPK was around level 5, I guess. Though last week they did have to use a Bead of Force to imprison themselves in order to survive the calamitous detonation of the Fortress of Three Sorrows, also in the Githyanki city.

The Room
You can see a map of this floor of the dungeon here. (There are another two maps associated with this dungeon, albeit much smaller.) One thing holding me back from some of [MENTION=20323]Quickleaf[/MENTION]'s excellent ideas is that this is room 7 of a 60 room dungeon, including the crypts and Duthka'gith creation labs. As asked by [MENTION=6801670]Istbor[/MENTION], the players do not know where the Phylactery is, but will need to clear the rest of the dungeon to do so. They entered the dungeon via a teleport from rooms 1 to 2, and have cleared rooms 2, 3, 10, 11, and 12 so far.

There is an unhallow effect in play, which you can see on page 3 of this document, which I wrote to update the rules of the dungeon.

Charles Rampant said:

Wow, lots of great responses here! While I absorb them, here is some extra detail as requested by posters.

Statblocks for the Tl'a'ikith and the Kr'y'izoth

Party Equipment
My vague wording in the OP has caused some confusion on this front. When I observed that they had no magic weapons, what I meant was that inside the anti-magic field they'll be lacking them, which is vitally important against a Lich who is immune to normal ones.

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If Anti-Magic removes magic weapons it really ought to remove immunity to non-magic weapons IMO. Otherwise a creature like Vlaakith can just cast it herself and become invincible. I can't see why monk hands should be any different, either. I'm sure the intent is just to remove the "plus" bonuses.

Oh, and as to the Antimagic Field, as asked by @Hemlock, the view I'm taking (in accord with the players - we sat and discussed it for like half an hour before they entered the dungeon) is:

Things that work

* Monk's magic hands - not a spell, an innate ability.
* Paladin and Cleric bonus weapon damage, on same logic as Monk hands.
* Lich's melee damage, on same logic.
* Paladin smites, as I understand from previous threads that it works fine in the field.
* Dragons, including breath weapons, since that's a different kind of magic.
* Int-based flight, as that's due to the astral plane.
* Monk's stunning fist and other base-class abilities - they're not affected by Magic Resistance, so it seems weird that anti-magic would affect them.
* No monsters will die just from entering the field, unless their statblock explicitly says so.

Things that Don't work

* Magic items, and all abilities derived from them - changes AC for the monk, for example.
* Spells.
* Githyanki psionic abilities - 5e seems to treat them as just spells with a different source. This one is crucial for the players, since otherwise Vlaakith can just misty step away from the field, so we clarified it early.
* The unhallow effect. This, combined with the Paladin smites working, means that actually the Paladin can do a lot of damage in melee, if the players try that route.
* The monk's elemental abilities.

Oh, that Unhallow effect is great. It means that if you can knock out the Paladin, everyone pretty much instantly has to make a DC 20 Charisma save or be frightened. That should really mess them up.

I think Magic Jar x2 (with Programmed Illusions to stand in for the Kry'i'zoths, so that they still draw Antimagic Fields and waste PC actions) + focus fire on the paladin + one guy who Shoves Vlaakith away from the grappler is definitely the most evil way to go here. Since Vlaakith is evil, that makes it the most Vlaakith way to go.

Does the paladin's Aura of Protection work inside the anti-magic field? It's important because it affects the monk's concentration saves and/or the concentration saves of whoever tries to AMF the cleric. (I assume nobody will be grappling the Kry'i'zoth, only Vlaakith, because the PCs are already short enough on action economy as it is.)

Watch out for that Horn of Valhalla--that can be a lifesaver for a party. If necessary, have the dragons breath fire on everything in the room to kill off the berserkers, even if that means that Kry'i'zoths need to burn a Legendary Resistance on minimizing the fire damage--it's more important to reduce that swarm of berserkers. (And hopefully the Kry'i'zoths will be wearing the Arcane Trickster's and the Champion Fighter's bodies anyway.)

BTW, why True Polymorph for Vlaakith and not Shapechange? Shapechange is more versatile in case you e.g. want to switch breath weapons. Is it because True Polymorph has no "you can't use any legendary or lair actions" clause? If so, note that the 6th printing of the MM applies that clause globally to all shape-changing. See http://dnd.wizards.com/sites/default/files/media/MM-Errata.pdf page 11. So, you might as well use Shapechange.

Charles Rampant said:

Enemy Plan
My baseline plan was for Vlaakith to use True Polymorph on herself (not Meteor Swarm, which the party are expecting) and to try to use an Ancient Silver Dragon's breath weapon to paralyze the whole party, especially those that normally avoid Red Dragon breath weapons with evasion. If that failed, she'd at least have loads of free HP. Once knocked out of that shape, probably Forcecage would be my next step, but I'm wary of it since the party cannot do anything about it - no arcane caster so no friendly short range teleport options, and it isn't even concentration. It seems likely to be unfun, so I didn't want to open with it. The two spellcasters would just use Finger of Death and whatnot to try and chip away at the party, and the Dragons would wade into melee.

As to the party's plan, it seems to me to be very devious. The Dragons are unlikely to fail against Banishment - since they have +11 on Cha, even if I ignore their Legendary Resistance as they're really not the boss here - but the Antimagic fields are the killer. Once those are cast, I can see only two routes for Vlaakith and her minions: either fight in melee to try and break concentration, or use Counterspell at the moment of casting to shut it down. Having her do Counterspell would almost certainly work - the party can't cast that themselves - but seems like a bit of a dick move to arbitrarily negate what is actually an interesting tactic.

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Just curious, but how is using a silver dragon's paralyzing breath more fun than using counterspell/forcecage combo? Sure, the party gets a save every round against the paralysyis but a DC24 Con save is pretty hard and the cone is certain to hit everyone. A player waiting every round for his turn just to roll a save and miss can be just as frustrating as being shut down by a spell. At least with forcecage it won't likely get the entire party and they have options to use range attacks or misty step out.

Don't get me wrong, I think using true polymorph to turn into an ancient silver dragon is a cool and sound plan. I'm just not sure it will make the battle more fun. And that's a lot of dragons to contend with, which may make combat kinda repetitive. Just some thoughts for you to consider.

My advice is go for it, full out. The players and characters knowingly attacked her even though they could have gotten what they wanted - this is on the PCs. That doesn't mean try to kill them, but it does mean they intentionally want to initiate at these odds and do something glorious (or fail trying). That's the player wish, and cheesing out the combat removes player agency.

Use counterspell. Personally, I would only allow it against the clerical one - I can't see a ring of wishes being obvious about casting the spell. Just like if you cast it with sorcerer's subtle spell, theren't nothing obvious to counterspell. Use forcecage as well, which can be easily broken by an antimagic field - if they are willing to move away from a caster to get to it. I love Faustian choices for my players.

I often design my encounters quite deadly for my PCs - and then I'm their biggest cheerleader and "say yes" enabler at the table. The consistently surprise me. You'll find foes stunned by your monk, denied casting by antimagic field, and all sorts of fun.

Actually, my biggest concern is that it's going to turn into a grind with all of those HPs, not how deadly it is. That PCs may end up dropping to lesser foes after the peak of the battle because they just can't do enough damage to take everyone out.

Minor question: is the resistance to radiant magical, will it also be gone in the antimagic field?

Evil Overlord Rule #40: "I will neither be chivalrous nor sporting. If I have an unstoppable superweapon, I will use it as early and as often as possible instead of keeping it in reserve."

Go full throttle right out of the gate. If your antagonist is fighting for their life, have them use everything they have. You don't need to check for a pulse if they're ash.

Can True Polymorph be used to polymorph an undead creature into a living creature?

Quartz said:

Can True Polymorph be used to polymorph an undead creature into a living creature?

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It doesn't specify. Just says "creature", so I don't see why not. Never thought of that application before. Definitely a lot cheaper than a true resurrection spell. However, the polymorphed creature still retains its alignment, so a good human transformed into an evil skeleton and polymorphed into a human would still be evil.

I think that Forcecage is more unfun than the Silver Dragon simply because the party has zero answer to it. If Vlaakith casts it, the party has absolutely no way of ending it on the affected members, since it has no save and does not require Concentration, while they lack the main answer to it (Disintegrate or teleportation spells) due to the party's lack of a real arcane caster. It's probably about as effective in the long run, but I feel that the one which lets them at least roll dice and use Greater Restoration spells to deactivate is the more entertaining option for everyone.

I didn't even know that the Shapechange spell existed. It seems like a somewhat better version of True Polymorph, but without the permanent duration. I might use it instead - the Concentration is the thing that'll limit it, not the actual spell duration itself, I would imagine.

The Fright effect from the Unhallow is one that I explicitly mentioned to the party on their arrival, so I'm looking forward to reminding them if the Paladin goes down or they get separated (e.g. if he gets Mazed). It's a fun idea! Meanwhile, the Horn of Valhalla I believe they've kept in reserve, so they should have it available. It'll be interesting to see if they decide to change plan and go for that instead, which would make the combat even grindier than it already will be, but also lets them control the situation with a mass of bodies. (I've already found that combats at this level are slow, simply because you have to use so many Gith Warriors to make it a challenge - so I expect this combat to be a whole session affair anyway.)

I like [MENTION=20564]Blue[/MENTION]'s suggestion of using Counterspell only on the Clerical version of Antimagic Field. That seems appropriate - and also preserves the level 9 slot for my own use, while I sacrifice the level 8 slot to guarantee the counter. Plus the players will probably try and force them both through at the same time, since Vlaakith only has the one reaction, while the Kr'y'izoths are unable to guarantee the level 9 Counterspell.

Summation: I think that my plan remains mostly the same - cast Shapechange with Vlaakith, toss spells with Kr'y'izoths, let the Dragons go wild - but the responses here have made it clear that holding back with counterspells and the like wouldn't feel right. The players did, after all, have the free opportunity to walk away from here with a victory in their wider goals, so I should give them both barrels in response to their gamble. I think that what'll actually matter the most is the first initiative roll, and what the first actors do - if Vlaakith wins, then it might be all over, while the Monk blowing the horn or anti-magic fielding will change everything. I'll try to remember to come back and report what happened, since I think that it'll be a fairly interesting fight all in.