Dungeons & Dragons (and the vast majority of both table top &/or role-playing games) use dice to advance the plot. In competitive games this levels the playing field a bit: skill in poker tends to guarantee a win, but the luck makes a difference. In D&D, the better your game-master is at balancing the encounters means the higher probability that you will gain a party wipe, the death of your entire adventuring troupe. Technically, if the game is played 'fairly' (whatever THAT means), it is only a matter of time until tragedy catches up. If the DM is human ('lacking Darkvision'), then this only speeds up the inevitable Domino Of Death for your group. D&D is a long and imaginary game of Russian Roulette.
Here is a list with brief descriptions. Find one you like. Go down to the point you like and read it. Got confused? Buy a map. Stare at it. Find out where you are and where you want to go. When you get there, come back here and finish reading some really fun ideas for party wipes.
Ghosts traditionally are lingering spirits that have a reason to stay. Should your group feel creative you can always swap in World of Darkness rules & lore. To sum up: you can give players fetter-totems, physical items of great emotional appeal that binds them to this world. If you like you could have them enchant up to their charisma score bonus (minimum of one) as a common magic item that stops them from wandering off to the Great Astral Beyond.
The downside to this is that this is hard to weave into your campaign. Ghosts have very personal motives and this new found perspective on the value of life might trivialize your Big Bad Evil Guy or Gal.
These things are CR5 and are impossible to kill, requiring a Wish to get them to move on. These undead just hot-swap bodies if any one of them breaks. Technically, killing a CR15 Mummy Lord is far easier. Though you can make their Team Revenge theme very campaign driven and story-focused, you throw out the meaning behind any dice rolls almost instantly. The right group of these can take out a Lich, a Tarrasque - or virtually anything else. Sure, you can repeatedly ask your players to stay in character but giving them indestructible characters might be something you may soon regret.
Technically, once you house the soul-spirit in some magic item, any steed would do. A skeleton is more intelligent, cheap to make and in this case the players may well have provided a body. In the back of the DMG you find that they get +2 Dex, -4 Int, -4 Cha. Remember, with your high-stat origin bodies your skeleton can have intelligence or charisma as high as 16, which can be weird. They do not communicate verbally, but they can learn sign-language (no rule against it) and technically the Message (cantrip) gives them the ability to occasionally whisper... in silence ('specific beats general'). The down-side is that characters are no longer 'playing' per se - and their (future?) life depends on this relatively fragile porter getting the job right. Also, you have to quasi-home-brew in the control players have over their newly minted dead body.
Alternatively you could give players a Flesh Golem &/or Shambling Mound for them to play with - say that it is a composite made (mostly) of their dead bodies, some weird accident / occurrence / experiment. This is obviously more powerful but it could work if players each control a limb or have to roll a charisma + skill in order to take control for a spell.
You can park players in nearly any magic item as 5e has utterly no rules on what it takes to make an intelligent item. Even a 'common' magic item seems to have enough magic-juju for keeping their soul from drifting into the Great Beyond - though they may not be able to talk. You could rule as a DM that they could get up to 'uncommon' enchantment if they spend enough time at focusing their will &/or collecting magic items - use rules from the DMG or Xanathar's.
As complex as this one seems to be, it appears to give players a lot of options for their recovery and can do great for enforcing team-work and cleverness.
This has a lot in common with the 'ghost' idea, above but has a lot less drawbacks. Sadly, it is much harder to implement because this is a high level and specialized spell. The good news: if you plan this correctly glyph of warding will work quite well. This does mean that your players will become semi-permanent residents in their newly found loot. It also means that death is a much more serious game, possibly upping the stakes somewhat.
Come Back... Again! Reincarnation Rocks
Reincarnation is is a mere 5th level spell - low enough to put into one of Tasha's tattoos if you like. You will notice that the material component requires NO GEMS. This means any community can grow, farm and collect all the components, typically at a rate of 5 g.p. value per person per day. Imagine a community that takes a day out every week to drum up oils & unguents ('other oils') for any fallen hero they may favour. Those that are chosen can have a special doll made, a common magic item that preserves their hair, blood or whatever else that identifies people that deserve restoring. When any of the objective-persons die they have ten days to cast the Reincarnation (on the doll-item) and bring this hero back. As someone. Should you need to buy more time, Crawford Tweets suggest that Gentle Repose works to give +10 days on the casting window.
The advantage of this system is that you can pull the players right out of the deadly situation. You also give them a brand-spanking-new body which is handy if they got aged, lost important parts - or got eaten / thrown in lava. The downside is that this usually requires planning, a not-trivial sub-plot and if your players know they have a Bookmark of Brand-new Body they will get unusually reckless with their adventuring. You can 'punish' players by requiring payback (quests, promises made in a Zone of Truth and more). You can also make it a real threat - once players use this once, allow a few of the bad NPCs to use this as well. Also of interest: the Wish version of Reincarnate requires no requirements. That means at the very least one does not need the 'unguents' nor the humanoid body bits. Does that also mean one has a 10 day window? Can an ancient and long dead wizard-king be brought back with a non-stress version of Wish? That is up to your interpretation - or rather, The Master Of The Game gets to make that choice.
As mentioned above, the 3rd level Revivify can have its cast-time extended via Gentle Repose - up to +10 days per casting. That means you could have a trivial one-use magic item (Common? Possibly Uncommon?) that gives Gentle Repose to the wearer upon their death. You could even have a Gentle Repose water or powder or oil or gunk that players apply, adding tension and spice to your combat rounds as everyone dies horribly. Imagine the tense rounds as they are all dying off - trying to ensure everyone is covered. Then the fun part: players have to play the NPCs as they try to get in there and rescue the bodies before they are turned into barbecue or cold-cuts. At least the body protects against animation as undead - there is that.
To 'punish' players you can remove limbs &/or organs. There are common magic items listed in various books for replacing any limbs... even eyes! Incredible prosthetics, assuming one can find a 3rd level character with the right formula / recipe (a non-magical yet uncommon item, RAW) to build it. One also requires the components from some situation or monster of CR 1 to 3.
This one requires some serious home-brew. Lycanthropes are immune to most damage, but only when the curse-disease takes effect. Before this body-enchantment lunacy takes place, players would still succumb to all the usual non-magical slashing, piercing and bonking as usual. Should you use this motif you need not even explain what happens to them. Each character will wake up after-the-fact in some weird place, alone - and naked. You are welcome to give hints like 'blood on your nails that is not yours' and 'really weird stuff in your mouth'. Remember, the stand-in lycanthrope had to get up and finish off the battle wherever players abandoned it by screwing it up. Did the lycanthrope beasts simply leave? Did they kill everyone? You get to decide as Master o' Dungeon.
You can make this curse as difficult or as fun as you like. In fact, if you give them something benevolent like Were-Bear or even the Were-Crow from Strahd's world, you can have this 'curse' stay silent through any number of deaths. This could make for a very weird puzzle, especially if many NPCs &/or monsters start becoming terrified of this indestructible group you have.
In the other realm there are supposed to be non-religious yet deeply religious humanoid pseudoanthropomorphicisms (say THAT ten times fast!) that shuttle the spirits (elves), souls (humans) and other psyche (all other humanoids &/or beasts &/or anything sentient... that isn't a Mind Flayer). Depending on your world one can negotiate your return in a Bill & Ted's Bogus Journey kind of way. Also seek inspiration from the Diskworld series if you like.
Should one of the party be a cleric or warlock or even a druid, paladin or ranger... bard, maybe?... you can negotiate something with a divine force. Typically deals of this kind are horribly expensive. Spoiler Alert: in the Strahd-verse this return-to-life is built right into the campaign. In video games like Fable II, you can die as often as you like and only gain a scar, sometimes. You can make this lifetime warranty as flexible as you choose. But as critics pointed out in Fable 2, once you remove all the sting of death the game combat becomes arbitrary and a bit pointless - so beware.
The concept Deus Ex Machina means that some ultra-powerful force comes along at the end of the story and just... fixes it all. Not only does this ruin the story, this makes the players feel utterly powerless, making them (technically) victims to a DM's NPC. It does not matter if it is a friendly god, a happy ancient dragon or even a passing medusa that has a back-storage of basilisk vomit to bring 'curiosities' back later on - cheating death is called 'cheating' for a reason.
If you do include the Ultimate Support Team - or any of the other devices listed above - keep this in mind: anything that is player-choice is NOT Deus Ex. I put that in both bold and italic for a reason: this is a salient point that makes the entire D&D game. If something happens that is players' (& their characters') choice, it is good. Anything players put into action, big or small, adds to the story and the plot. This is why they put dice into the game: players declare their choices and gamble to see if it works out. In a nutshell, this is the entire game. You will find that other guides point out: any character's death (and staying dead) is good if that's what works for that player.
