In second edition D&D, something like Leadership took shape. Certain classes - namely the fighter, ranger, and cleric - attracted followers once they hit 8th to 10th level. The fighter had a series of charts you rolled on to determine how many people showed up and what they could do. One character led the troops (the equivalent to a cohort), while the other, lesser followers were men at arms. The fighter could only attract these people if she had a stronghold and some land, but it was pretty clear what the fighter was supposed to do with her followers.
The Leadership feat in 3rd edition (and iterations like Pathfinder) is more complex; it includes multiple bonuses and penalties and leaders continue to attract followers throughout their career. Since it offers access to characters with their own stats and abilities, it also has more potential for abuse. But for all of its nuances, the writers remain relatively silent about appropriate classes and races, and what followers are supposed to be for. For all of these reasons, the Leadership feat is placed squarely in the hands of Dungeon Masters - who don't always want to be bothered because of the feat's bad reputation. But it doesn't have to be a lot of extra work to tailor Leadership for your campaign, and it can introduce a lot of fun to your game.
The first step to implementing Leadership involves developing a concept of how the feat is going to function in your game. Followers are great for delivering plot hooks and introducing NPCs, as well as making the world come alive with recurring characters, but they can deliver quick mechanical benefits, too.
They are ideal for keeping leaders connected to local events and for taking care of things that leaders leave behind, like strongholds. For example, you could allow leaders to choose one or more of the following bonuses:
For every ten followers present at the location, it gains a +1 morale bonus versus infiltration or attack.
For every ten followers tending a business during the month, the leader gains a +1 modifier on the profit check (see DMG II, Chapter 6).
For every ten followers helping with preparations, the leader receives a -1% to the total price of creating a new stronghold (see the Stronghold Builder's Handbook Chapter 1).
If followers are the staff at your stronghold, you can pay them up to 50% less in wages (see the Stronghold Builder's Handbook Chapter 2).
For every ten followers who are members of the same affiliation, the leader gains a +1 modifier to their affiliation score (see PHB II).
If followers are going to provide mostly social support, consider making skill bonuses available when the leader has access to their followers. For instance, for every ten followers that are consulted or enlisted to help, the leader can gain a +1 circumstance bonus to a skill. Some skills might not be appropriate for this bonus, such as physical skills with Dexterity or Strength as their primary attribute (like Climb), those that put someone one-on-one with an animal (like Handle Animal and Ride), and Use Magic Device.
If followers are going to create items for the leader to use, you could limit access to a maximum of 50 gp worth of items per 10 followers per month (or 25 gp, or however much you find reasonable). You can restrict the output to mundane and alchemical items, or include magical scrolls, since the feat can be taken at first level.
If you want the leader to be able to utilize followers in combat, consider these alterations:
For every ten followers used as ranged support during the round (providing cover fire and doing little else), the leader gains a +1 morale bonus on their next ranged attack.
For every ten followers used in total defense (meaning they don't attack but maintain a shielding formation), the leader gains +1 shield bonus for the round. If they move more than five feet away from the defensive formation, the leader loses the bonus.
For every ten followers engaged in melee combat during the round, the leader gains a +1 morale bonus on their next melee attack.
If you don't want leaders to constantly have to refresh their pool of followers, but you do want to show that the little guys can be injured, treat followers as a group. Each follower in the group has 10 hit points; the group has a +5 to all saving throws. An attack that hits at least AC 10 strikes the group. Enemies must target followers or areas that followers inhabit in order to damage them (which they might not bother to do, since there are bigger fish to fry). For every 10 points of damage done to a group of followers, one follower falls unconscious. They are not dead or dying, but they will remain out of the running until roused or until the battle is over. For every 5,000 gp spent per 10 followers, they gain +1 to AC (or +1 to saves) when acting in a group. The leader should be able to influence or outright control followers' decisions in combat, in order to make this effective.
You could also allow the following combat options:
For every ten divine spellcasters providing support services on the battlefield, the leader may call upon up to 1d10 first level divine spells during the round (as per a first level caster). These spellcasters cannot provide any other kind of support during the round. Spells can be called upon for as many rounds as half the leader's level.
For every ten arcane spellcasters providing support services on the battlefield, the leader may call upon up to 1d10 first level arcane spells during the round (as per a first level caster). These spellcasters cannot provide any other kind of support during the round. Spells can be called upon for as many rounds as half the leader's level.
For every ten barbarians raging on the battlefield, the leader gains +1 morale bonus to Strength and Constitution for round. These barbarians cannot provide any other kind of support during the round. This ability can be called upon for as many rounds as half the leader's level.
For every ten bards providing support, the leader can call upon up to 1d10 bardic songs available at first level. The songs last for the round. These barbs cannot provide any other kind of support during the round. This ability can be called upon for as many rounds as half the leader's level.
For every ten paladins engaged in battle, the leader can add +1d10 damage to their next melee attack, provided it hits, as per smite evil. These paladins cannot provide any other kind of support during the round. This ability can be called upon for as many rounds as half the leader's level.
For every ten rogues providing backup on the battlefield, the leader may add +1d6 to their next attack (providing it hits). These rogues cannot provide any other kind of support during the round. This ability can be called upon for as many rounds as half the leader's level.
Tying followers closely to their leaders is meant to streamline the combat process and to keep the influence of the Leadership feat reasonable. With more effort, you can arrange to pit followers against other followers in large-scale battles, completely separate from the PCs.
To keep things simple, you might allow a leader to use his followers for one major function (location, social, item creation or combat) and that's it. Or you could allow a leader to choose one bonus from each category.
Based on your decisions about function and the power level of your game, you will have to decide which classes followers can have. The 3.0 DMG states that followers can only be warriors, commoners, or experts, but the 3.5 DMG removes that restriction. NPC classes offer fewer options all around, particularly with spells and special abilities. Since most followers won't get beyond first level, however, they will only be able to do so much damage, even with PC classes. Followers can be great for experimentation and showcasing - you can give them classes that you don't normally use or classes that your players might never consider taking.
The DM will also have to decide what the distribution of classes will be like. Is a paladin more likely to have clerics and other paladins as followers? Will only 1/4 of the followers be spellcasters? A lot will depend on the setup of the campaign, the leader's class, location, and ties to others. Followers might be from the same church, guild, or secret organization as their leader. Followers can also be attracted depending on what the leader does. Saving a village can make the people there very grateful, after all. If you want a more nuanced experience, you can decide these things per character; if you want a simpler route, you can decide them for the whole game.
You'll also have to decide if you want to give cohorts access to prestige classes. This is an important decision because cohorts are meant to be higher level followers and they will probably end up adventuring with the party. You could prohibit cohorts from taking prestige classes altogether and keep it simple. You could also decide to take it on a case by case basis. If a cohort is a part of an organization with their leader and there's a related prestige class, then it might make sense for the cohort to have access to that class.
Recent versions of Leadership leave race open, but restrictions might apply. Some races have such high level adjustments that they're pretty much out of the question, except for high-level cohorts. Some might not be available due to location or other concerns. First, decide if followers and cohorts can come from nonstandard races, either for one leader or for the whole campaign. You might also want to look at monster levels from Savage Species, as they could be useful for a plane-hopping campaign.
Leadership is usually based on Charisma, to represent powers of attraction. This makes Charisma important to anyone who wants Leadership, and not just to sorcerers and bards. Charisma, however, does not have to be the only option. If you are running a game in a setting like Oriental Adventures, you might want to base Leadership on a character's honor and/or family reputation. If a monk has developed a reputation for her amazing Wisdom, you might want to allow her to base her Leadership score on that.
Additionally, you will have to decide if you want to alter the number or levels of followers. More followers will cut down on person-to-person interaction; fewer followers will allow leaders to get to know their people better. When a leader gains a level and is due to gather more people, you can choose to let them level up the people they already have. For example, in my game Thorik the dwarven cleric had 10 first level followers for a while. When his Leadership score increased, I gave him the option of having 15 first level followers, or adding a level to five of his current followers. He decided to level up 5 of his well-known followers to second level, rather than waiting for 5 new first level people. It worked well and did not shift the balance of power in a significant way.
You should pause to consider the penalties and bonuses that apply to attracting cohorts and followers, since they can say a lot about the type of game you're running. If you are running an evil campaign, for instance, cruelty might not be a bad sign to an evil follower. Instead, you might substitute a -2 penalty for misplaced mercy, because being soft on enemies bespeaks a soft leader. You could remove the word "aloofness" if followers expect a leader to look down on them, but keep a penalty for "arbitrariness," since followers want to know what to expect. (And you should consider whether or not slaves can count as followers, if slavery is an aspect of your evil campaign.)
When you're done with bonuses and penalties, you should also think about keeping track of a cohort's experience. If a cohort is already the maximum level they can be, then it only creates more work to keep track of experience. If a cohort might be able to gain a level, then the extra work is worth it (but should be handled by the player). Just as importantly, you should decide if you will allow leaders to have multiple cohorts. You could decide that a leader can use leftover cohort levels to attract a second cohort, or you might decide that one NPC in the party is enough.
When a character takes the Leadership feat, the player will have to calculate the character's Leadership score(s). There is a separate score for cohorts and followers. These scores will let you know the maximum number of NPCs the leader is entitled to. Problems often arise when Dungeon Masters leave too much up to their players. Many players take advantage of the feat (and the DM) by choosing each follower so that they yield the maximum possible benefit and cover all of the leader's weak areas. Many players will also build their followers and cohorts as stacked as they can, if they are left in charge of their follower's stats and gear. Powergaming players aren't the only ones who will abuse the feat; the temptation is often too great, even for fair players. Needless to say, these choices can greatly unbalance the game.
Even if you trust your players a good deal, you will have to decide if you want to leave followers and cohorts up to them. As the DM, you can manage things with all due attention to game balance, and you can surprise your players with the details of different NPCs. As the DM, you also know how much you are willing to give your players - and it is very difficult to take something back once it's given. You could always allow the player in question to devise some NPCs and then randomly choose which ones are used and which are left aside. You might also allow other players in the group to create a couple of followers and randomly choose some of those. Think about it carefully, in any case.
DMs should also think carefully about who gets to portray cohorts, in and out of combat. Generally, cohorts will be involved in the PCs' exploits and will have a voice in the group, unless they are effectively ignored. The DM could allow the leader's player to portray their cohort during all situations, and the player might want to, but the cohort will probably feel like less of their own person and more like an extension of the leader. The DM might opt to play the cohort instead, either during combat or all of the time. Or, the DM could portray the cohort but allow the leader's player to control what the cohort does during combat. For a fun twist (and if the group is willing), allow another player to portray the cohort.
Once a character takes the Leadership feat, you have to ask yourself how they will gain their followers and cohort(s). Just because a character has leveled up and claimed the feat, that doesn't mean that NPCs will start tracking them down or appearing out of thin air, unless that makes sense for your purposes. Pre-existing NPCs - friends, siblings, and so forth - make great cohorts and followers. It will help cut down on the work load (and make the most sense) to choose from NPCs the leader already knows. You can then present these NPCs in a new light. You will have to decide how much say-so a leader will have in the process. Just because a leader wants to attract a cleric, that doesn't mean a cleric will be interested. If leaders have a lot of control they might make choices that will be problematic. If leaders have no control, however, they might feel like the feat wasn't worth taking.
DMs will have to decide how to balance this, since leaders will be entitled to new followers with each new level they obtain. Will you allow leaders to actively recruit people? How successful will their efforts be? If you want leaders to gain followers quickly and easily, then allowing them to have very successful recruitment drives can help. If, however, you want to give the impression that NPCs are people, then you might want to complicate things. Just because a leader calls out for followers, that doesn't mean things will go their way. People might be put off, especially if the leader has a spotty reputation. NPCs might decide that they don't want to leave their home for a person they don't know, or they might be reluctant for some other reason. This is not to say that the leader should always be thwarted, but a monkey wrench every now and then can remind the leader that followers follow because they want to, first and foremost.
A streamlined but distinctive version of Leadership is in the third chapter of my drow book, if you're looking for something different.
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