If it's just for Fly that you need another spellcaster, then instead you can use Potions of Flying.

I don't have a character sheet for Onyx. Does he know the Guidance cantrip? It's normally a cleric/druid spell, unless Onyx has a feat or ability or something that grants him an off-class cantrip. And cantrips do not appear on spell scrolls. Otherwise, Quin might have to attempt her spell without Guidance.

Quin can attempt to cast Reverse Gravity as many times as required, but it would ruin the fun if there was no penalty for failure. I could make it so that any time she fails to cast the spell there is a cumulative 10% chance that the tarrasque wakes up.

Current plan as follows, assuming you use the potions (correct me if I'm wrong):

- Onyx, Lia, Hatch, Throm and Lok coat relevant weapons with Oil of Sharpness to make their attacks magical. They drink Potions of Flying and hover within range of a point 100' above the tarrasque. 

- Quin casts Eagle's Splendor on herself: adv on CHA checks. Onyx casts Guidance on Quin.

- Quin casts Reverse Gravity targeting the tarrasque's location, dropping concentration on Eagle's Splendor. (Both are concentration spells, but the rules say "You lose concentration on a spell if you cast another spell that requires concentration. You can't concentrate on two spells at once." To me that means you don't lose conc on the first spell until you successfully cast the second)

- The tarrasque presumably reverse-falls 100 feet. Lia, Hatch, Throm and Lok start shooting it or yelling at it with Thaumaturgy to wake it (if not already awake) and provoke it to anger.

- Onyx (having dropped conc on Guidance) uses Lannel to deliver the Fly spell on the tarrasque. The tarrasque presumably is a willing target as it wants to retaliate against Lia, Hatch, Throm and Lok.

- The tarrasque flies toward one of them. Profit.

(edited)

Throm will grab a potion of flying, no need for a cast spell. He does note that although he will be able to fly for an hour instead of 10 minutes, his flying speed will be his ground speed equivalent (30’/round) instead of 60’/round for the cast Fly spell. This means the Tarrasque will eventually catch him if the curse isn’t lifted pretty quickly.

(Is there a potion alternative to magic weapon specifically for a crossbow, akin to oil of sharpness? It would be nice if no one had to concentrate on keeping my shots magical to hit the Tarrasque.)

(Just reread the description of oil of sharpness. If we have an unlimited supply, I’ll coat all of my bolts (60, why not?) and I will have +3 for up to an hour with no concentration needed.)

Of course, I forgot about that. Galerius already mentioned taking one. OK, so Oil of Sharpness for anyone that wants it, negating the need for Onyx to cast Magic Weapon.

(have edited the above plan)

Quote from ZorgAlmighty >>

If it's just for Fly that you need another spellcaster, then instead you can use Potions of Flying.

(( Doh! Forgot you'd given us equipment too! That fixes that. ))

Quote from ZorgAlmighty >>

I don't have a character sheet for Onyx. Does he know the Guidance cantrip? It's normally a cleric/druid spell, unless Onyx has a feat or ability or something that grants him an off-class cantrip. And cantrips do not appear on spell scrolls. Otherwise, Quin might have to attempt her spell without Guidance.

Quin can attempt to cast Reverse Gravity as many times as required, but it would ruin the fun if there was no penalty for failure. I could make it so that any time she fails to cast the spell there is a cumulative 10% chance that the tarrasque wakes up.

(( There's a link to Onyx's sheet for this campaign in my signature; in case it isn't working: https://ddb.ac/characters/29069854/x6b6J4

He started out as a warlock who took Pact of the Tome at level 3... so 2 warlock cantrips at that point, plus three from any class he wanted thanks to his Book of Shadows. (Of course he'd grab Guidance!) Got a third warlock cantrip at level 4. Right afterwards, at level 5, he took a level of sorcerer, so that was 4 more cantrips. So yeah, while I hadn't planned it that way, he actually wound up with 10 cantrips by 5th level.

I figured he would have found a few ritual spells along the way too and tossed in three more first-level spells into his Book of Shadows, since he took the Book of Ancient Secrets invocation (allowing recording rituals from any class.) ))

Wow, look at all those cantrips.

OK, so Guidance is back on the menu. Will amend the plan again to include it.

(( That... looks ideal. ))

“I love this plan! I’m excited to be a part of it! Let’s do it!” Throm is mysteriously channeling Peter Venkman from the original Ghostbusters.

"How do I apply Oil of Sharpness to my crossbow/scimitar?"

(I assume you wipe it on your blade or dip your crossbow bolts in it)

Quote from Jfw6369 >>

"How do I apply Oil of Sharpness to my crossbow/scimitar?"

 However your imagination dictates. Go crazy.

 I'll allow everyone a chance to comment before we proceed. I feel the end drawing nigh in dramatic fashion....

Throm feels the urge to quickly check on Kardinya just to be sure that an iteration of Delmon is not harassing her.

Kardinya continues to go about the business of keeping the castle clean and the patrons fed. Despite looking as busy as always, she seems unusually calm amidst the growing sense of anticipation about confronting the monster upstairs, which has inevitably permeated through the castle. Many of the patrons are apprehensive (or downright terrified) of what might happen should the tarrasque awaken and go on a rampage, particularly as no-one is able to leave. But Kardinya's aura of calm and tranquility helps to allay the worst of the tension. Willagee seems as jovial as usual, and as much his normal self as Kardinya. Only Bentley seems different from the bumbling halfling you first met in the tavern, and sniffs with a haughty turned-up nose of disapproval at the "rabble of tourists who seem determined to upset the status quo" as he says.

(I'm still trying to figure out how Kardinya was able to hit Delmon with an unarmed strike and do 1d4+12 damage. She must be a 4th-level monk with some serious DEX buffs.)

Onyx checked his scrolls. Two of Fly, one of True Polymorph, one of Conjure Fey, one of Banishment, and even one of Hold Monster that he had grabbed on a lark. Hopefully they would all be unnecessary, and the Reverse Gravity would work on its own; but as far as he was concerned, it was better to have some sort of backup rather than to curse the sun. Fly first, then the others as the situation seemed to dictate.

A Potion of Supreme Healing, and a Potion of Psychic Resistance, also hopefully unneeded, but thankfully, still suggested.

And just about then, Lannel landed on his shoulder. Onyx smiled at the creature, who cocked its head to look at him and gave a terse nod.

"Alright, Lannel," he quietly said as he crossed his arms across his chest, each hand making the same eldritch sign. This wasn't a spell, but rather pure elemental magic summoned directly from his patron--and with it, the warlock floated up several inches off the floor. Now slightly aloft, Onyx turned to his familiar and instructed him on the plan, to the extent that it needed to know for its part. "And since you know the way," he finally added, "when everyone's ready, go ahead and lead us up to the creature."

"So im to dip my dagger in this gelatinous oil? And that will make it magical?" Lok asked incredulously 

"If you say so..."

The barbarian shrugged and spent a minute applying  Oil of Sharpness to his dagger 

 "I suppose im as ready as im going to be" Lok stated, getting his things together and making sure he understood his part in the plan

"Mad Maggie best be prepared once this curse is lifted. Assuming I dont die horribly within the next several hours, Im after her next."  Lok declared, mostly to himself. 

Hatch applied Oil of Sharpness to all his axes.

"I'd like to say or do something before we head off to confront the thing, but I just can't think of anything. This is the best plan we got. It's either this or we live in this hotel forever. It's nice, but a prison is a prison. After this, whoever wants to go seek vengeance on the witch or goddess who put us through this, I'll join you."

Throm clears his throat, “gentlemen, ladies, no matter how this turns out, I want to tell you all it’s been an honor to serve with you. Although it is my hope we all end up back home, I have no regrets should I not make it.  Someone please take care of Flax, my pony, should I fall. He has been a worthy traveling companion and silent confidante for many years. You should find him still tied up to the hitching post outside the original tavern.” And with that, Throm readies his bolts with the oil of sharpness. He then eyes the stairwell and fingers the stopper corking the potion of flying.

Onyx listened to the others with a sense of admiration and a bity of envy. They'd been here far longer than him, and had clearly developed various bonds together. It was true, that this could be the end. And while he had been aware of that fact, it was the sort of thing he honestly never really focused on, and always tried to stay above. For him, the goal--usually, the knowledge--was all that mattered. But here he was reminded that it wasn't that way for everyone else.

"We've got to succeed," he thought, "for their sake, if nothing else."

While waiting for the “go” signal, a thought crosses Throm’s mind. He purposefully begins searching through his pockets. Not there ... not there ... not there ... ah, there it is. Out of a deep internal pocket comes the petrified “carrot” he had forgotten to give to Flax when he tied him up, how long ago was it? Now in a toast he raises the “carrot” and quietly says, “Flax, I swear, after this, the rest is yours.” And with that, he places an end between his back molars and bites hard with a forceful pull down. “CRACK” a sound not dissimilar to a bugbear’s femur being broken echoes throughout the room. Throm is glad he wasn’t on the roof yet. That might have been enough to wake the dead. As he chews, thankfully with his mouth closed because the sound is like chewing gravel, Throm thinks, “I hope we go soon, I’ll need that flying potion to wash this down.”