Is it good form as DM to include a video game-like "cut scene" in the game? Upcoming in my game I am planning on having an NPC that is travelling with my party betray them to open a portal to an astral demi-plane where an erinyes and a traitorous deva are being held in order to free them. I am think of just narrating the NPC opening the portal and the party does not get a chance to act to stop her. They will be able to act once the portal is open.  Is this good form or does it take too much agency away from my players?

If you think your players will enjoy it, do it. That is the #1 rule.

However, in my opinion, D&D is not a video game, and video game techniques like cut-scenes have no place at the D&D table. But that is my way of looking at it -- your job as a DM isn't to do what I would like, but what your players at the table like.

To expand on what bio wizard said, It would be upsetting to many players. I can easily see someone saying, but I have the alert feat/a 20 dex/ any number of other class, or racial or magical or item abilities, I should get a roll to be able to act before she finishes. And they would be right. 

Now, if the NPC is real sneaky, that can work. Maybe they do it when the party is too far away, or sneak off to do it during a fight, or at night when the NPC is the only one on watch, and the party wakes up to find it open, that kind of thing. If the NPC has been with them, she will know their strengths and weaknesses and can make a plan to exploit them. 

I have seen the cutscene thing work on Chain of Acheron, with Matt Colville as the DM. But he did 2 things to make it work. First, he gave them a "cutscene" of something that was happening "off screen" (the party was not there), and it was just a flavor thing -- a dark woman summoned another woman to her while the party was fighting some big bad guy, and said something like, "The opening moves have started, go do X." And that was it. So the cutscene was something the players could not have intervened with anyway.

The other thing he did was, he said, "Now we're going to have a cutscene." By telling them it was a cutscene, he let them know they were just supposed to listen and not try to do anything to intervene. His players were fine with it both times, and it did not seem awkward or forced the way an in-person (party is there watching) cutscene is.

Now, what you could potentially do is this: You could have the NPC make a surprise move, and ask for an initiative roll but declare (since there are no rules for determining this anyway and it is all down to DM's discretion) that you judge the party to have been "surprised." The rules for surprise state that the initiative order goes as normal but the surprised side doesn't get to act -- i.e., the side causing the surprise gets a full round to act and the surprised side can't directly respond. If you make opening the portal be an Action or a Bonus Action, then the NPC can do this on the surprise round, and then when that is over, the PCs can act on initiative order. You can still describe what you are doing in a florid, cutscene-kind-of way if you want, but just make sure it only takes one action or one bonus action to open the portal.

Additionally if you judge the NPC to need 2 rounds, then IMO, for this one purpose, fudge the rolls so she wins the initiative. This would give her 2 rounds... the surprise round, and then the second normal round, before the PCs can act. 

Functionally this is not a lot different from a cut-scene because you are rigging it, but if you call for an initiative roll and they just "lose" the initiative and it is a surprise round on top, the players are much less likely to complain than if you start narrating, they try to act, and you just say no.  They are used to having to sit and watch on the NPC's turn during tacked combat. They are not used to having to sit and watch when there are no turns and they should be able to act, but you just tell them they can't.

I use them very sparingly if it is something that is absolutely required narratively and I make it clear to the party that I’m narrating. I use them more liberally if it is inconsequential or just to move things along. Players can generally redirect or interrupt this if they want to do something. For example, if the story is flowing nicely, I might narrate a long trip if I feel it will interrupt things too much, but the players can do something along the way if they do choose. Another example might be as they enter a city looking for something specific where they’ve explained their strategy to me, I might narrate this if I don’t want to get into minutae.

This ties in a bit with the idea of only allowing a die roll if it’s consequential and if you can live with the results either way. “Consequential” has a bit more flex as you might be deceiving the party into think something could happen, but if you have to have something happen (or not happen) don’t put it behind an open die roll. Narration is one valid tool to handle this situation. 

Id agree with Lyxen make an in charcter reason they know it happens/happened but without them having the ability to do anything about it.

If the NPC is powerful enough to open a portal maybe have them also drop a paralysing spell at the same time. So the characters can see it but are helpless to do anything... maybe even have NPC taunt them about it to give them a bit more desire to get  revenge? 

Quote from GaspodeWD >>

If the NPC is powerful enough to open a portal maybe have them also drop a paralysing spell at the same time. So the characters can see it but are helpless to do anything... maybe even have NPC taunt them about it to give them a bit more desire to get  revenge? 

 The problem there is, someone will make their save against the spell, if you say there’s no save, we’re back to the original problem. 
They could, however, drop a wall of force between them and the party. Unless someone has disintegrate ready, they can then do what they like. 

If every Campaign book could include an INTRO cutscene.... there could be so many hype expected.

I suppose depends on party level and assumed bady level to what they villain could get away with and the players having no counter...

I think for the scenario indicated, you just need to give the NPC a surprise round or even just use their turn. Especially if the party isn’t expecting anything. Action: open portal.

If they don’t know about the betrayal, it’s unlikely they’d try to block the spell. If they are fighting, that’s a different thing. If you do it with the party present, you’d better plan for them coming up with something that would stop things  

Maybe they find the portal after the fact or maybe it doesn’t matter whether it’s opened at that moment. I think you have lots of story options that don’t require a cut scene. 

I don’t have as bigger problem as others here with the cut scene idea, like your the DM setting up the encounter with description is the job. I would say try and make it sound different to how you would set up an interactive situation, get the detail in to try and get your players invested in listening to you rather than thinking about their next move. Here is the thing the whole of D&D isn’t played in initiative order and combat time, the encounter starts when you say it does as the DM. If the cutscene of the portal opening is what kicks off the encounter interaction then to me that’s fine.

just because it takes you 5-10 minutes to explain it doesn’t mean it takes more than 20 secs in real time

You can also have the NPC set up a situation where the party would be distracted anyway.  That makes the desired action of the NPC to open said portal w/out the party being able to swallow it easier.  A glamour bard who neither looks like a bard and who doesn't play a conventional instrument, for instance.   It's even one of their core abilities.  Or create a hostage situation involving another NPC at the other end of a large room.  Or the opened portal takes place in a room a floor directly above the PCs, but they can see it because the ceiling/floor is transparent, maybe even make it a thick layer of glass that breaks and falls on the PCs once the portal does open.  There a dozens of different ways to separate the PC's position or attention from what the NPC is doing.