Pfthb

My Adventurers League group is playing though Curse of Strahd and we will be fighting Strahd next session. I'm playing as a diviner with the Lucky feat, the spell Tasha's Hideous Laughter (which will be referred to as THL), and a bag of holding.

Now my plan is to give most of the party scrolls of THL so we can all cast that spell. Statistically, with several people casting that spell, and me basically giving him disadvantage, he will probably fail his save against the spell which will incapacitate him. Then one of the other PCs will stab him with a wooden stake. Next turn we put him in the bag of holding. Then all we need to do is infiltrate his castle and find his resting place, or stab the bag of holding (with mage hand or a ten foot pole) and banish him to the astral plane.

Could this work?

I see a couple of problems with this plan:

1. The size of a bag of holding. While the bag has internal volume sufficient to hold Strahd, getting him inside may prove impossible.
   
   

   
   

   This bag has an interior space considerably larger than its outside dimensions, roughly 2 feet in diameter at the mouth and 4 feet deep.

   
   

   
   


2. Strahd, like most "boss monsters" has legendary resistances. 5 Castings of Tasha's Hideous Laughter is unlikely to be enough to incapacitate him, even with portent dice.
   
   

   
   

   Legendary Resistance (3/Day): If Strahd fails a saving throw, he can choose to succeed instead.

   
   

   
   


3. The feat lucky isn't going to apply here, as it only affects attacks against you and your attacks, not saves against your spells.
   
   

   
   

   Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20 ... You can also spend one luck point when an attack roll is made against you.

   
   

   
   


4. I wouldn't count on Strahd not having a way back from the Astral Plane. He's a powerful spellcaster, a vampire and has made many deals with the dark powers.

According to page 24 in Curse of Strahd (using this related answer):

No spell - not even wish -- allows one to escape from Strahd's domain. Astral Projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail...

A Bag of Holding may not be a spell, but reading a bit further...

Magic that summons creatures or objects from other planes functions normally in Barovia, as does magic that involves an extradimensional space. Any spells cast within such an extradimensional space (such as that created by Mordenkainen's Magnificent Mansion) are subject to the same restrictions as magic cast in Barovia.

As such, banishing him to the astral plane doesn't seem like an option.

There are several problems

1.) Nothing Leaves Ravenloft

A central problem in Ravenloft is that it is all but impossible to leave that plane. Relevant text can be found here (Curse of Strahd, p. 24, bold added):

No spell - not even wish - allows one to escape from Strahd's domain. Astral projection, teleport, plane shift, and similar spells cast for the purpose of leaving Barovia simply fail, as do effects that banish a creature to another plane of existence. These restrictions apply to magic items and artifacts that have properties that transport or banish creatures to other planes.

As such, the bag of holding would not be able to transport a creature to another plane of existence.

2.) Heart Stabbing is not a given

If you could stab a creature in the heart whenever it was incapacitated automatically, then THL would be a save-or-die spell for most creatures. 5e doesn't have specific rules on "called shots", but there's some guidance given, with relevant text on hit points in the PHB p. 197:

When you drop below half your hit point maximum, you show signs of wear, such as cuts and bruises. An attack that reduces you to 0 hit points strikes you directly, leaving a bleeding injury or other trauma, or it simply knocks you unconscious.

In order to drive a wooden stake through Strahd's heart, you'd need to reduce him to zero hit points. Normally, this is done by attempting to stake the vampire when it is already at 0 hit points, which brings me to my next point.

3.) Timing Matters

Note the relevant text on staking a vampire (MM p. 297, bold added)

Stake to the Heart. If a piercing weapon made of wood
is driven into the vampire's heart while the vampire is incapacitated in its resting place, the vampire is paralyzed until the stake is removed.

It's not enough to have the vampire in its resting place, incapacitated, with a stake through its heart that was driven there earlier: you must drive the stake through its heart "while the vampire is incapacitated in its resting place." Unless Strahd was somehow in his coffin when you (successfully) cast tasha's hideous laughter on him, these terms would not be satisfied by your strategy.

4.) Spell Scrolls only work for certain people

Although any person can read a generic "magical scroll" to attempt to decypher its meaning, a spell scroll is a specific magic item with its own rules. Specifically (DMG, p.200):

If the spell is on your class's spell list, you can use an action to read the scroll and cast its spell without having to provide any of the spell's components. Otherwise, the scroll is unintelligible.

Since tasha's hideous laughter is a first level spell, I'm assuming that any character in your party who has it on their class's spell list could cast it already without a scroll. But anyone who doesn't have that spell on their class list will be unable to use the spell scroll to cast it.

Provided you could fit a creature or object inside your bag of holding, your best bet would be to toss in a Portable Hole (Into the bag of Holding).