Time Stop and Acererak (Spoilers Obviously)

[NecessaryWeevil]

So we just finished Tomb of Horrors from Tales From the Yawning Portal.

We encountered Acererak, and our Bladesinger blew him away with a misread Time Stop. But we thought it was a cool ending and let it stand. The question arose, "What is this spell good for?"

So, out of curiosity...

You're a 17th-level Elven Bladesinger. You've found Acererak in his little chamber and provoked him into attacking. You've won initiative and decided to open with Time Stop. You roll the d4 and get 4 turns out of the spell. Using the spell as written in the PHB, what are you doing with those four turns?

[If it matters, your other other party members are a Tabaxi Shepherd Druid, a Gnome Arcane Trickster Rogue, and a half-Drow Brute Fighter with a few levels in Rogue. The Druid has a staff of the Woodlands and the entire party has various ways to fly using their gear. The party is at full health but you and the druid have expended about 30% of your spell slots. Nobody is concentrating on any spells. You can make reasonable assumptions about spells prepared.]

[Nagog]

Cast as many non-concentration buff spells as you can on yourself, and assuming you have it, upcast Shadow Blade. Then, do everything you can to get into melee range of Acererak. I'm not familiar with his stats myself, but if he's build like any other spellcaster, once you're in melee range, the fight may as well be over, particularly if you have Mage Slayer. Keep your Counterspell handy and prevent any and all teleports. Wizard battles are 100% decided on how many resources you're willing to commit and how quickly you're willing to burn them.

Spells that may come in handy:

COUNTERSPELL(Obviously)
Mordenkainen's Sword: Bonus Action attack, deals good damage, and it's Force damage! However, it is concentration.
Flesh to Stone: Another concentration spell, but it targets Constitution (which Wizards don't typically have) and basically has them rolling them like death saves. 3 failures and they're Petrified, which is about as good as dead.
Magic Missile: Pretty dang good for breaking concentration, but with something as (assumedly) low AC as Acererak, Scorching Ray may be better, particularly as Shield will automatically negate MM.

[Valmark]

So we just finished Tomb of Horrors from Tales From the Yawning Portal.

We encountered Acererak, and our Bladesinger blew him away with a misread Time Stop. But we thought it was a cool ending and let it stand. The question arose, "What is this spell good for?"

So, out of curiosity...

You're a 17th-level Elven Bladesinger. You've found Acererak in his little chamber and provoked him into attacking. You've won initiative and decided to open with Time Stop. You roll the d4 and get 4 turns out of the spell. Using the spell as written in the PHB, what are you doing with those four turns?

[If it matters, your other other party members are a Tabaxi Shepherd Druid, a Gnome Arcane Trickster Rogue, and a half-Drow Brute Fighter with a few levels in Rogue. The Druid has a staff of the Woodlands and the entire party has various ways to fly using their gear. The party is at full health but you and the druid have expended about 30% of your spell slots. Nobody is concentrating on any spells. You can make reasonable assumptions about spells prepared.]
I probably wouldn't cast Time Stop in the first place- a slightly buffed Demilich is likely a medium encounter at that level, even with the aid of the ghost (if triggered).

That said... There's no real space to throw one of the delayed damage spells and it's immune to the damage of the useful buff spells I can think of so I'd use a Summon Fiend- out of the summons with ranged attacks (not enough space) that's the only one Acerarak isn't immune to.

Assuming a magic rapier (a +1 is plenty, just to bypass the immunity) to this you should have a +12 to hit so 65% accuracy (I'm considering you maxed out Dexterity and Intelligence). Discarding crits 24 damage comes from you before using a bonus action, meanwhile the Fiend does 43 damage (on average) if casted at 8th level. Then get away (triggering an AoO since it doesn't do much damage) and let the Brute mop up. Or whoever else, dealing the remaining damage shouldn't be hard.

If you don't have a magical weapon pulling out Elemental Weapon (which you should have if you need magical weapons) let's you deal 40 average damage, but then you're likely better of throwing it on the Brute so that it can leverage an Action Surge for six attacks all with +3 to attack and +3d4 damage.

I wouldn't spend slots on buffing myself since by now you should have done the whole dungeon and a demilich is really weak by this level in defensive terms, but otherwise the best use for Time Stop is to buff and throw a Delayed Blast Fireball.

Small question- what was the rest of the initiative order? The fighter could probably finish the fight by itself but otherwise the demilich has a good chance of knocking both the druid and the bard out of the fight with Howl- you should back away big time in order to not risk it.

If the Initiative is set up like that it could actually be worthwile to throw a DBF and blast your party and Acerarak regardless- Acerarak doesn't have good Dex so you have a good chance to destroy it by stacking it (remember that it's 15d6 since you can build it for three rounds, not 12d6) with Summon Fiend or an Elemental Weapon round.

Important note: Elemental Weapon at that level takes a 7th level slot so you'd use an 8th level one for DBF, so the damage is actually 16d6.

Cast as many non-concentration buff spells as you can on yourself, and assuming you have it, upcast Shadow Blade. Then, do everything you can to get into melee range of Acererak. I'm not familiar with his stats myself, but if he's build like any other spellcaster, once you're in melee range, the fight may as well be over, particularly if you have Mage Slayer. Keep your Counterspell handy and prevent any and all teleports. Wizard battles are 100% decided on how many resources you're willing to commit and how quickly you're willing to burn them.

Spells that may come in handy:

COUNTERSPELL(Obviously)
Mordenkainen's Sword: Bonus Action attack, deals good damage, and it's Force damage! However, it is concentration.
Flesh to Stone: Another concentration spell, but it targets Constitution (which Wizards don't typically have) and basically has them rolling them like death saves. 3 failures and they're Petrified, which is about as good as dead.
Magic Missile: Pretty dang good for breaking concentration, but with something as (assumedly) low AC as Acererak, Scorching Ray may be better, particularly as Shield will automatically negate MM.

Take a Demilich and remove lair actions, then add a DC 19 (I think) Cha save action to remove a player from the fight. That's Acerarak (from Tomb of Horrors, the Tomb of Annihilation one is far stronger).

So it'd be immune to Shadowblade and pietrification and doesn't have spellcasting.

[Osuniev]

My memories of Acererak's statblock are fuzzy, but this is what I would do with a Timestop :

To start with, I probably wouldn't open with Timestop if I knew the fight was happening : I would have cast the buff spells and drunk the potions beforehand.

I would use Timestop mid fight, to heal myself with supreme healing potions and renew spells I would have lost concentration on.

But assuming your scenario :
I would probably cast Fireshield, drink a potion of invulnerability, or a potion of haste, summon something, and finally attack. The kind of thing one does before the fight, but that I didn't get to do before.

[Osuniev]

Flesh to Stone: Another concentration spell, but it targets Constitution (which Wizards don't typically have) and basically has them rolling them like death saves. 3 failures and they're Petrified, which is about as good as dead.

As a DM, I wouldn't rule that a Demilich is made of Flesh. Even for a Lich, I probably would give them advantage on the save, or something.

[Lord Vukodlak]

As a DM, I wouldn't rule that a Demilich is made of Flesh. Even for a Lich, I probably would give them advantage on the save, or something.

So if I cast flesh to stone on a human I get a statue filled with human bones? Jokes aside Acererak is specifically immune to Petrification