I am planning to design a Adventure which has a strong political flavor and is taking place in the Nine Hells. Its gonna be an evil campaign. I plan to let the heroes start at level 15 and require the group to be lawful, as long their Character has no background provided, that clearly prevents them from turning against each other. So they at least have a chance to withstand the dangers of everyday life that the Nine Hells are bringing.
But when I started designing all the political parties and hidden forces being involved into the plot, it started reminding me a bit on the first 4 seasons of Game of Thrones. And I started realizing that a single bad decision might lead to forces turning against the party; even a group of level 20 heroes is unlikely to survive. I mean, it's the powerplays of archdevils that the group will be taking part in.
I could mitigate this to some extent by a NPC that is a spy for the opposing political party getting intel from the group, becoming not a spy anymore, and some yet unknown NPC being the plot-relevant spy and such alike. But other situations like siding with the wrong forces in a battle taking place might inevitably lead to a TPK, as I can't see how I could reshape such a complex plot just on the run.
Given that wrong decisions are likely to lead to a TPK, how can I ensure that the players still have an continuous experience of the adventure, despite being very limited in recreation of new characters? As the characters are required to have specific intel about the plot and what the previous party figured out to be going on hidden in secrecy.
I could imagine that a new group simply could be a special force of one of the Lords the party was working for and simply being briefed about what they need to know. But I fear that being dispossessed by the freedom to design their characters background makes it feel not like their character anymore from the moment the first TPK happened.
Best Answer
Keep in mind that this is an answer to the general question of dangerous campaign and some aspects might work less well with evil and inherently selfish characters. Also I freely admit that I took some parts from the other excellent answers.
Make their deaths meaningful
Think of any book or movie where one of the main characters dies. If it is a good book, this is almost always done not to annoy the reader but to advance the plot or to emphasize some things. Examples that come to mind are:
All of them have some sort of hook that motivates further actions of other characters, in those examples
Even a death by bad luck can motivate whole epics. Eurydice died by accidentally stepping on a snake. Still Orpheus quest to resurrect her is one of the most famous Greek myths.
The question is how to translate this into an RPG. You cannot simply kill of characters to advance the story at a predetermined point as this will make your players rightfully hate you. Instead you'll have to put in some more work. Prepare your main plot as if everyone will survive and then at every point have a backup in hand in case the party dies and that ideally makes their death meaningful. Those backups will of course alter the main plot, so a lot of the preparation will never be used, but may be salvaged for later backups.
You also mentioned specific intel that characters might have and that might get lost. I would not consider this a bug but a feature. Tell them that some information needs to be returned at all costs and watch how some will fight to the death to allow the crucial NPC to escape. Or send in the next group to find the remains and retrieve the notes they gathered. Or have another group encounter their killer with the plot-relevant artifact. Or maybe since you are in the lawful part of the afterlife, take on the herculean task of obtain Permit A38 in order to retrieve their souls from the unhelpful stygian bureaucrats. Also some metagaming might be to your advantage. The characters in the new group might not know that the big bad flattened the old one without breaking a sweat, but their players will surely not try the direct approach again.
Personally I would go with alternating between multiple groups. Possibly two or three main ones. This way you will not have to start from scratch every time. You can also do fun things like cliffhangers and last minute rescues, where you stop the action of group A right before their inevitable death and continue with group B three days earlier, when they get tasked with counter-ambushing the ambush that is about to happen three days later. If a TPK happens early in the session, a group switch to something unrelated also allows you to finish the session without having to ad-hoc the details of you backup plot.
You can also fit in some one-offs, to lighten things up with some unrelated story-fluff or to emphasize the dangers by actually putting disposable characters onto an actual suicide mission. They could even be sent on that mission by one of the original groups, which makes the players both care about their success and less worry about their inevitable death. Or if they actually survive, you'll have a backup for your next TPK.
There are a lot of fun possibilities here, but on an important note, expectations matter. Tell your players about your plans beforehand. Specifically tell them, to get attached to the story and the setting, not to the characters and that while you'll never kill by an act of god, you won't save them from stupidity, bad luck or any of the other dangers in the setting and that you expect them to die, quite possibly more than once.
But please note that I never played a full campaign in the style, it is just how I would run something like this (and after writing everything down, I definitely feel the itch to do so). But the general idea is based on stuff that is known to work, namely death as a plot hook (ported from other fiction). I just combined it with another time honed classic, the extremely dangerous one-off side adventure and took it to a logical conclusion.