I am not buying the new Planescape books -- I have enough D&D content I'm not using currently -- but I think this is a pretty neat concept:
D&D Beyond said:For the adventure Turn of Fortune’s Wheel, you’ll need to create a 3rd-level character and two variants of them, or incarnations. While your character's incarnations may have different species, classes, and abilities, they are all still one character. Over the course of your travels in Sigil and the Outlands, your character may be replaced by one of their other incarnations.
D&D Beyond said:The multiverse is a vast and complicated thing in Dungeons & Dragons, but even it can’t seem to make sense of you. In Turn of Fortune’s Wheel, you’ll be playing what’s known as a glitch character. A glitch character is made up of three incarnations, which are basically different versions of the same person.
(I couldn't find this discussed in the 45,000 pages of the 200 existing Planescape threads. Sorry if this has already been hashed and rehashed.)D&D Beyond said:Glitch characters suffer from amnesia. When the adventure kicks off in Turn of Fortune’s Wheel, you’ll find your character struggles to remember who they were and where they came from. But, as you’ll start the adventure at 3rd level, it’s clear you’ve been to places and done things. Part of the joy of Turn of Fortune’s Wheel is rediscovering who you are, for better or worse. This gap in memory presents ample opportunity for roleplay and for devious DMs.
When you die, you are replaced by another incarnation. Look, the multiverse doesn’t know what to do with you, so your death gives it an opportunity to set things right. Unless someone acts fast to bring you back from the dead, one of your other incarnations will emerge to take your place. Keep dying, and you’ll find yourself swapping between your three incarnations like a merry-go-round of death.
Luckily, all your incarnations level together, and they’re automatically attuned to whatever magic items you were attuned to in your previous incarnation. But they may be down spell slots or class features, depending on what your DM decides. So, there are certainly pros and cons to being nigh-unkillable.
Incarnations appear in strange places—and at strange times. Dying is rarely convenient, and that remains true in Turn of Fortune’s Wheel. It may take minutes, hours, or even years for your incarnation to show up; the multiverse is hard at work at bringing you back, and that takes time! Luckily, that serves as an opportunity for someone to cast raise dead on you. If they move fast enough, you won't be replaced by another incarnation.
Chances are your incarnation won’t just waltz into the chamber where you were facing down a lethal threat, either. You may later find them wandering down a desolate alleyway or sitting in an empty room with no recollection of where they came from. On that note, if everyone in your party happens to die, that could spell disaster depending on how long it takes for your incarnations to appear. But we’ll leave those consequences to your DM.
This may also explain the level 17 portion of the campaign -- the action picks up with a significantly more powerful glitch character, although that breaks the rules as laid out above. A time jump also makes a lot of sense, or player characters getting powered up by a dead god's essence or something.