Kaodi
Hero
This is a kind of wacky idea for a campaign that I just thought of while thinking about solo games: Under the effects of a curse, all player characters are fused into one being who randomly comes under the control of their different personalities. Outside of combat they can choose which personality is in control (though they may at times get stuck with one at the DMs discretion, and some mechanic for disputes), but during combat the stress causes them to randomly switch personalities every round, and thus abilities. Or, alternatively, all personalities roll for initiative against each other and go in that order until everyone has gone once. The other thing is, while they may have a combined hit point total (that is probably really high), individual personalties can be killed by taking too much of the right kinds of damage; psychic for sure, maybe necrotic as well. I am not sure how you would deal with personalities dying, like maybe the curse will cause other potential characters to be absorbed from time to time, but I am sure some inventive way to deal with it could be arranged.