‘The Adventuress of Henrietta Street.’ introduces Sabbath as a reoccurring villain to the Doctor Who novels. A former member of the British Secret Service Sabbath was a ritualist and an engineer eventually travelling through space and time on board his metal ship, the ‘Jonah’.
In many ways he was the equal to the Doctor, similar to the Master. What sets him apart was that Sabbath’s motivation was to protect the Earth. In a universe without the Time Lords Sabbath saw himself as the protector of his home planet and resented the Doctor, seeing him as being at least partially responsible for the dangers the Earth faced.
Several of the novels would feature Sabbath at odds with each other but both trying to solve the same problem. The way they did this was often incompatible, causing further complications as they frustrated each others attempts to find a solution.
The root of this disagreement is that Sabbath was much more ruthless than the Doctor. He was quite prepared to kill if it ended a threat, even if that person was innocent. Like the Master he would also make alliances with hostile aliens if it furthered his goals.
Sabbath was also capable of great guile and manipulation. He could be charming when he wanted to be, persuading others to do his bidding and when that failed, tricking them. On several occasions he saved the Doctor’s life, because keeping him alive suited his purposes.
Creating an opponent like this for your player characters changes the normal dynamic for an adventure. In a standard game the player characters will just have to stop the villain from achieving their goals.
When the opponent is like Sabbath it isn’t simply enough to stop them, since their goal is the same as the player characters. The player characters must foil their plan and also put a better plan in place.
It is worth bearing in mind that if the opponent is familiar with the player characters he will also be trying to stop them. This could be as simple laying a trap for them or sending people to attack them but it could also include betrayal, tricking them and revealing their presence to the enemy.
The opponent might keep the player character’s alive to steal their resources of use them as a distraction. On rare occasions they might actually form real alliances, although whether the player characters will trust them is another matter.
When crafting such an opponent they should have attributes and skills that are the same point total as the player characters. They should conceivably be able to do the same job as the player characters and be able to go toe to toe with them if needed.
The major difference will be how they go about achieving their goals. This should be the opposite of the way the player characters usually get things done. If they are subtle the opponent should be overt, if they use violence then the opponent should use charm, if they favour science then the opponent should have arcane skills.
Such an opponent should have something recognisable about them so their presence is unmistakable. They could be very fat or very thin, have distinctive hairstyle or wear unusual clothing. You’ll want something that as soon as you describe that aspect the player characters will immediately recognise it as the opponent.
If you want to use the opponent on a frequent basis the campaign will either need to be limited to a single time and place or the opponent will have to have his own mode of transport.
A rival Time Lord would fit this category well but you could follow the Sabbath route and have them build their own time machine. This would make them a menace to the web of time and even more of a reason for the player characters to try and stop them.
You might give the opponent their time vessel after they’ve encountered the player characters a few time so it is a surprise when they encounter the opponent in a different era. For example an investigator might encounter the player characters from the 1970s to the 1980s and build their own time ship from studying their TARDIS or an alien race might gift them with their own vessel due to their efforts.
If the player characters get into trouble it can be a good opportunity for the opponent to appear and save them. If the player characters owe the opponent then it reinforces the fact that the character is trying to help, in their own way.
You can change the nature of the opponent over time. If they are continually frustrated by the player characters their motivation might become darker, moving towards true evil. They could decide they want the player characters dead or humiliate them by engineering a scenario which they can’t solve.