Does this mean Charisma measures some kind of magical aura that inspires people to loyalty or fear?
Surprisingly, Richard of Richard's Dystopian Pokeverse answered, "yes." This got me thinking, especially since none of the standard saving throw categories lend themselves nicely to morale. Since Holmes says of Charisma:
A character of charisma below 13 can not hire more than 5 followers, and their loyalty will be luke-warm at best — that is, if the fighting gets hot there is a good probability they will run away. On the other hand, someone with a charisma of 18 can win over a large number of followers (men or monsters) who will probably stand by him to the death.
what if the morale saving throw were directly linked to Charisma? The target number on a d20 could be determined with the following formula:
Morale = [Character's Charisma + level] - HD of the monster.
Any pluses in a HD (such as the Hobgoblin's 1+1) are added to the roll. Monsters would have to roll over the target in order to make their save and henchmen would have to roll under.
For example, let's take a 1st level fighting-man with a 12 Charisma and his henchman fighting a band of Lizard Men with 2+1 HD. If the henchman had to make a morale saving throw, he would have to roll under an 11 on a d20 while adding one to the roll. The Lizard Men would have to roll over an 11 while adding one to their roll.
Assuming all things are average and equal, a character with a 10 Charisma would have a 55% chance of a henchmen failing the morale saving throw and a 50% chance of a 1 HD creature failing the morale saving throw. This falls right in the percentile range of a 6 or a 7 morale (58% and 41% respectively) using the 2d6 morale rules in B/X.
What I like about this formula is that it makes both Charisma and HD more meaningful. An Ogre is less likely to run away from a group of 1st level adventurers than a group of 5th level adventurers — unless one of those Charisma scores was 5 better than the Charisma scores of the 5th level group.