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In AD&D 2nd edition as well as 3E, there were rules for characters who were lower than 1st level. These “0-level” characters were like pre-adventurers, meant to symbolize children or apprentices. I’ve played in a couple of games that began with us playing zero-level characters who would then develop into 1st level characters. For my taste, 1st level characters are already incredibly weak, famously vulnerable to the attack of a house cat in some editions.
But I was thinking of a way to address some of that vulnerability. In 5E, of course, every character chooses a background which grants them proficiencies, equipment, and often some kind of story-based ability. I realized that it would be a logical step to let the background also provide a hit die.
Most of the sample basic NPCs in the MM have two or three hit dice, making them slightly tougher than some first level player-characters. And the impression I’ve always had of D&D is that characters aren’t really adults until around 3rd level. This is reinforced by the fact that in 5E, you usually don’t choose an archetype until 3rd level.
One option here is to just let each background grant the d8 hit die that medium-sized creatures get, or a d6 in the case of a small-sized PC. I kind of like the idea that the d10 is reserved for mighty heroes, fighter-types, and that the d12 is the boss die. Possibly even bawss. But, if you don’t want to just use a d8, here are suggestions ranging from d4 to d8, based on how tough I think a given background would make you.
Folk Hero: d8
Guild Artisan: d6
These hit dice, whether variable or d8s, should function the same way that normal hit dice do. As to whether you roll them, or take the average, or take the maximum, just do what you’d normally do. If you want PCs to have lots of hit points at the beginning, take the maximum. Otherwise, roll or average as usual.