I was brainstorming ideas for a campaign and thought it might be fun to include an interlude in which the PCs actually visit a strange land of the dead. There they would go on a minor quest to solve certain tasks in order to re-enter the world of the living, this time armed with special magical tokens gifted by the gods themselves.
The plan is to have them encounter the BBEG or a top henchman, lose the battle, and be spirited to another plane.
On one hand I think this could be an exciting, unexpected outcome. On the other hand, I have trouble coming up with the best way to lead up to this. I want the players to have their own agency in the world and I'm grappling with whether or not it's okay to force this outcome. I don't want them to feel like all of their choices leading up to the battle in which they die didn't matter.
Is there a way I could let the party organically wind up in this circumstance or perhaps change the circumstance in some way?
I am not a fan of the "meant to lose" encounters. They might work in video games, but in the context of FRPGs they can really break your rapport with the players. I might consider pulling an Edgar Rice Burroughs on this one - instead of combat, have the players exposed to something like a gas that makes them black out. Maybe one the bad guy led them into. When they wake up they are on the outer planes. This leaves ambiguity as to whether they are dead, got moved while unconscious, or are astrally projecting, etc.
This way you can keep player trust. Just be sure that none of the PCs have anything like a necklace of adaption or periapt of proof against poison, and set teh save against the gass Ridiculousy high (DC 25 or so) and required at the end of each turn when they end it within the area of the gas, so any PCs still awake have a good chance of passing out trying to save his comrades.
Other options might be drugged food, a bizarre spell, or some kind of possessing force that requires the ever-so-underutilized Charisma save.
I wouldn't force a loss on them. Be ready for them to win the battle, but also if they lose, that's when they could be sent to another plane.
If only a few PCs die, you could have a portal open, and the surviving members hear their companion's voices calling from beyond...
If you're running a plot-based game where the players are expected to pursue the storyline to its finish where they do battle with the villain and the players know this, then it's a simple matter of telling them of your idea and asking for their buy-in. If you're running a sandbox game where the players can roam about the world engaging with your metaplots or exploring as they please, then that might be a tougher sell because you're asking them to buy into a plot when you're not running a plot-based game. Either way, you're going to have to be upfront with the players about it. If they like your idea, then they'll probably go for it. If they don't, shelve it for some other time. If they don't buy into your idea and you try to force it anyway, then you're railroading and that's subverting the will of your players which is generally not taken very well.
As for the scene in which the characters will lose the battle and die, you'd have to frame that in the beginning of the scene: There is no hope of survival. This is the characters' last stand. They can make a difference if they succeed (prevent the demon invasion, hold off the gnoll army long enough for reinforcements to arrive, or destroy the doomsday device, etc.), but they will die in the doing. When you play out the scene, you're not playing to see whether the PCs will live or die - that's already decided. Instead, you're playing to determine the result of some other set of stakes. Once you've gotten past that bit, off you go to Hades or wherever for new adventures.
So that's it - just seek your players' buy-in on the idea, then govern yourself accordingly. Good luck!
Thank you all for the ideas. You've all given valuable advice. I could make the event trigger be something other than combat-related death such as a poison. I also like the idea of a portal or perhaps some psychic message delivered to the living PCs from beyond by the PC that got himself or herself killed. Ideally the characters would willingly sacrifice themselves as an option.
Clueing them in early could be helpful too. I'd like the afterlife thing to be a surprise but I could ask each player to roll a wisdom perception check before some event. Those that pass would get the sense that the action would lead to certain death. I could foretell this death through prophecy and give clues throughout the campaign.
I recommend not offering choices that aren't actually a choice or rolling dice when the desired outcome is known. If you want a thing to happen and you involve dice, there is a chance that what you want to happen won't happen. Clues may not be understood by players. Hooks that lead to the unwinnable battle may not be followed. You want a very specific outcome to occur and you're hoping that subtly manipulating four or five other people at table will get you there. That is a risky proposition. You're better off, in my view, just telling them what you have in mind and seeking buy-in.
You could also think of a scenario where a Big Bad does something horrible and the PCs pursue ... right into a planar gateway trap. Then they have to take down the Big Bad and get back home. Toss in time shifting as well: sometimes the arrive in the other plane first, sometimes later. Maybe they get the jump on him, or vice versa.
Other planar stuff can be real cool. Good luck!
Or you could plan things where your players think they are going to beat the big bad and they do but the afterlife situtation is the big bad having a last hurray before they buy it.
I like the idea of the Afterlife adventure but would probably set it up as more of a stand-by adventure in case of a TPK, than as a predestined thing.
I think that as a player, if I had made some bad tactical choices and rolled badly and then died as a result, I would be pumped for a "Claw my way out of hell" scenario.
But if I had played well, rolled average or better, and still couldn't make a dent in my foe then I would start thinking something was fishy. And when the afterlife scenario popped up It would be very obvious that my choices didn't matter.
I feel the same about setting ridiculously high DCs for checks. If the gas is definitely gonna knock someone out then don't taunt them with a roll. At the most I might see what the PCs con mods are and have them collapse in the appropriate order. So the wizard faints straight away 
and the barbarian and fighter have just long enough to give each other a quizzical look 
before they follow.
You could start the campaign after they're dead.
Everything interesting that will happen in this campaign happens after the PCs are dead, right? Instead of building up to an avoidable, and possibly unsatisfying death, just start there instead. Point to your players and tell them, "Omae wa mou shindeiru." Let them know that the Ontological Mystery surrounding the circumstances and consequences of their deaths will be the premise of the campaign, and work from their.
If you want to give your players even more adjacency, I can think of no better way to get them invested in the idea than letting them fill in the details of their untimely death as part of their background. How did they die? Were they betrayed? Poisoned? An unfortunate accident that happen under mysterious circumstances? Were there any significant consequences to their death. Did they die a hero? A villian? Did they succeed or fail to save someone or something? What unfinished business motivates them to return to the land of the living? Reclaim your kingdom? Be there for your family? Destiny? Good ol' fashioned revenge? Did the party know each other before their deaths? Did they die together, or did their souls meet on the other side seemingly by chance?
If you still want the BBEG to be the common thread linking their deaths, just ask your players to include the BBEG in their backgrounds as well. Or not, and surprise them later. Either could be fun.