[RPG] What happens in Curse of Strahd if the players destroy Strahd’s coffin or prevent him from returning to it

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.

The Dark Powers have never been all that clearly defined.

There is no official documentation nor official statements I'm aware of that describe a whole lot about the nature and history of the Dark Powers. That makes this question difficult (if not impossible) to answer definitively.

I have a searchable copy of Domains of Dread from D&D's second edition which contains this section in Chapter 1 (the searchable copy seems to have some formatting differences from a print copy, so I can't give a page number citation). I'm not aware of any clearer or more definitive information from any later edition, so this is the best support I could find even for a question tagged for 5e:

Defining the Dark Powers

Is it possible, therefore, to learn the secrets of the dark powers through a careful study of the macabre place they have created? One would think so, but proving the validity of the conclusions produced by such a study would be difficult or even impossible.

The precise nature of the dark powers is debated by sages and scholars throughout the land. Sages have argued about it since the earliest days of Ravenloft's existence. Quite simply, no one seems to know the answer to this question. More importantly, it is impossible to say whether any given answer is right or wrong. The Demiplane's most educated residents have uncovered conflicting evidence, supporting any number of possible answers. Consider the following arguments:

The Dark Powers are Evil: Certainly, evidence exists to support this belief. After all, the dark powers have gone to a great deal of trouble to create a vast region in which to concentrate the evil of the multiverse. Could this be something of a recruiting center from which an incredible army of malevolence will one day be released? That possibility could be the most frightening conclusion one can reach.

The Dark Powers are Good: An equal number of scholars put forth the assumption that the dark powers are actually creatures of good, not evil. After all, they have imprisoned some of the most terrible beings in the multiverse. In fact, not only are these creatures imprisoned, they are endlessly tormented. Can any force that so punishes evildoers be considered anything but good?

The Dark Powers are Few: Some scholars believe it is incorrect to use the plural when referring to the masters of Ravenloft. It is their assertion that a single entity, whether good or evil, rules the entire Demiplane. This theory is especially popular among those who believe that Ravenloft was created for some special purpose. If this is the case, it is easier for them to believe that a single entity is working behind the scenes to coordinate these events.

The Dark Powers are Many: Equally or even more popular, however, is the belief that the dark powers are numerous. According to this theory, no single being could accomplish all that the dark powers have. Of course, exactly how numerous the dark powers are remains a subject of much debate. Some say that there are three, the number in a coven of hags, while others insist they are a legion. To date, however, no one has provided any evidence to support these claims.

The Dark Powers are Imaginary: At the root of all debates about the dark powers, of course, is the question of whether or not they even exist. Many say that they are nothing more than a manifestation of the darkest side of the imagination. To be sure, no physical proof of their existence has surfaced. While many people claim to have seen the dark powers, none of these reports are verifiable or even especially reliable.

The Dark Powers are Real: Of course, if the dark powers do not exist, it is difficult to explain the creation of the Demiplane of Dread. Most Ravenloft scholars agree that the dark powers, whoever or whatever they are, do exist.

The two statements listed in the question aren't hard to reconcile when we don't have many constraints on what the Dark Powers are, but the "correct" way to do so should be chosen to fit your campaign rather than to match a canonical explanation that doesn't exist. A few possibilities that sprang to my mind:

• The Dark Powers could easily be a group that has accepted new members over time, with a specific subset being relevant to Curse of Strahd but not describing the Dark Powers as a whole.
• They could be fundamentally a part of, or derived from, the Shadowfell, but have a specific manifestation on the Material Plane which coalesced in the Amber Temple at some point (regardless of anything else that may have been, or later come to be, trapped there).
• The evil vestiges may or may not have been some or all of the Dark Powers, either originally or through some later development.
• The Dark Powers may have a distinct manifestation among the Domains of Dread, and so have been "born" in the Amber Temple after Barovia was withdrawn from its original plane of existence.
• And, of course, what Exethanter "knows" about the Dark Powers and the Amber Temple may simply be incorrect despite his confidence in the accuracy of the one thing he remembers.