Here are a few ideas.
Give Them a Common Goal
A common goal can bring a group together and give them incentive to work together instead of working at cross-purposes. You want to make sure they have to work together to accomplish the goal. By, in effect, forcing them to work together they will come together and bond on the in-character level. Players bring their own goals into a campaign during their character generation, but by providing one for them it gives them another reason to stay together. You want to replace their own selfish goals with one that requires a group to accomplish.
One of the biggest downfalls of an evil campaign is the selfishness that can develop. A shared goal will eliminate some of this. Even a selfish player can find reasons to work with the other members of the group if they feel it will bring them closer to their goal, or in this case, the goal you provided for them.
In order to make this work the goal is one that the group can share in. If “there can only be one” to accomplish the goal you are in fact providing reasons for the party to backstab each other. Perhaps the end result requires a certain number of participants.
Example: The recovery and use of the ancient artifacts, the Five Dark Stars. These artifacts require each to be wielded by one individual and those five individuals must use them in concert. By themselves they are useless, but together they provide immense power.
Example: The party’s goal is to take over the drow city. In order to make this work, they will need to each take a certain area of expertise. One to deal with things on the diplomatic front, one to run a spy network, one to deal with magical threats, one to subvert the religious factions and one to run the military.
Avoid Splitting the Party
It’s an old adage to never split the party, but it is perhaps even more relevant in an evil campaign. I am not talking about having the characters split up between different physical locations, but rather in their goals. If you, as the GM, insert individual goals into the campaign, especially ones that cause the party to work against each other then you only have yourself to blame when the campaign ends due to backstabbing. Keep the party working together and all toward the same goals.
Likewise, do not insert adventures “that test the party’s loyalty to each other”. They will fail, or at the very least one will fail the test and that can be enough to end a campaign. This means, do not have a shadowy figure offer great rewards to betray the party.
Provide a Common Enemy
Players who participate in an evil campaign will want to have their characters do bad things. That is the major draw to such a campaign. If you do not provide NPCs for them to do bad things to, they will find someone and that someone is usually someone in the party. Give the players someone to be evil to; focus their badness into an area you want it to go and away from the party.
Give Them a Common Background
During character generation give do not let the players make their own backgrounds. Give them one that ties the group together. They could all be from the same family or organization. They could have had a mentor that brought them together. In fact, their mentor or organization could be a radical thinker who believes in a unified front and has ingrained these thoughts into the characters as they were growing up. The point is to make them already part of an organization that values group unity.
Make it Awesome
By this I mean, make the concept of working together appear like it’s the cool thing to do. Make working in a group a unique and unusual thing. Make it seem like it’s an advantage that the world does not prescribe to. Show how being a unified group gives the players an edge over their enemy or simply the world around them. For my drow campaign, I would highlight that the other drow are loners who never work with others unless they are only using them for a limited time. I would mention, and show, that the drow can never gain the upper hand completely because of the fact they always act alone.
Provide a way to resolve conflicts in game. If the party is part of a larger organization, make a system wherein they can take out their aggressions against each other in a non-lethal manner. Perhaps the organization has a formalized system of non-lethal combat if there is a dispute between its members. Players tend to run evil characters that will brook no stain on their honor or reputation. With a full group all doing this it will inevitably lead to conflict. Providing a non-lethal and readily understood method of resolving these conflicts will allow all the characters to “save face”.
Involve the Players
Ask the players for reasons why their party will be able to work together. This does two things. One is it will each player a rational why they don’t go solo or backstab the other arty members at the fist opportunity. Second, it brings them into the big picture. By letting them decide how and why their character is able to work within the group they are more invested in making sure that is the outcome. If a player is having problems coming up with some ideas, I have a few below. If each PC takes one you can create a web of ties between them.
- His life was once saved by one of the characters.
- Working with one of the characters pays off a debt he owes someone else.
- The character has a mystical bond with another of the characters (perhaps the same birthmark or he feels non-debilitating pain when the other is injured).
- Sibling (though this only really works if the setting provides strong ties between family members).
- A prophecy/omen/fortune-telling/godly voice has told the character that one day the other character will save his life. The reverse of this is that the voice told him that if the other character dies he will die soon thereafter.
- An item/ability that the other character has and is only usable by him gives you more power.
Using some or all of these ideas can help keep an evil campaign going strong. Do you have any other ideas that can instill party loyalty in an evil campaign?