The damage conversion and the Fray die ensure that the PC is much slower to go down during combat and much more dangerous toward monstrous foes. Every round, the PC will usually do at least 1 HD worth of damage to any enemy of equal or fewer hit dice thanks to the fray die, and has a chance of doing more with their regular attack. A hero with a +1 weapon or a +1 modifier to the damage roll is guaranteed to do at least 1 HD in damage, and a fighter with a great-sword might even do 4 HD.

A typical novice fighter with 5 hit points and AC 4 from chain mail and a shield might run into a melee with four orcs during his exploration. Every round, his fray die can be expected to kill one of them, with a 1-in-6 chance of killing two. With a 1d8 sword in hand and a +1 Strength bonus, the fighter has an excellent chance of delivering 2 points of damage with every hit, leaving it possible that a lucky round might kill all four orcs at once.

Every round, the orcs can take a swing at the fighter, doing from 0-2 points of damage on each successful hit. It’s a dangerous situation for the fighter, but he needs to be hit at least three times before he’s in peril of his life. Compared to a lone hero using the usual rules for old-school combat, the fighter will probably win the engagement rather than being rapidly reduced to stew meat by orcish spears.

Against more fearsome foes, things get uglier for him. Against an ogre with 4 hit dice and a 1d10 club, he can’t use his Fray die because the ogre has more hit dice than he has levels. Furthermore, the ogre’s club is much more likely to result in the high damage roll that would inflict two or four points of damage on him. Still, it won’t take more than about four hits for him to bring down the brute if he can stay alive long enough to deliver them. If he were to confront the beast at second level, with another 4 or 5 hit points of his own, the odds would be much worse for the hulking creature. A fourth-level solo hero would chop the brute down in two or three rounds, with his Fray die helping the monster to its well-deserved grave.

Instant-death poisons, death spells, inescapable pit traps, and mind-controlling sorceries that would leave a hero helpless are best answered by Defying Death. That particular rule is meant to take the finality out of “save or die” situations by converting a failed save into damage instead. Because the number of dice scale evenly with the level of the character, there’s no level at which a PC can just wave away these threats- they’re always going to eat about one hit point for each PC level, and if doom is dodged too often at the table it becomes more and more likely that the bigger dice are going to be returning two or four points of damage each.

These solo rules are intended to fit PCs for classic adventure without any need to change either them or the written module. A single canny 1st level thief with a good hit point roll and a little caution can dare the perils of Quasqueton with a reasonable hope of survival, and a 3rd level fighter with a stout sword arm has a decent chance of standing fast against the perils of the sand-swept Lost City. If you use the Enduring Protagonist optional rule, you can start a game with a first-level PC and still have reasonable expectations of seeing high levels with the hero.