It was the White Guardian’s distress call that led the Doctor to become involved in the Eternals boat race and the Black Guardian had already sided with Captain Wrack, giving her the tools to sabotage the other competitors.
For those who wish to run an epic series of adventures the Guardians can provide a good catalyst. A request from the White Guardian suggests that the player characters must be highly thought of if such a powerful being is putting the fate of the universe in their hands and no villain can be more intimidating to face than the undying Black Guardian.
There is much mystery surrounding them. What we know is that they are immortal. Even when the Black Guardian is seemly destroyed the White Guardian explains that he will return, his powers undiminished and that they will both exist until the universe no longer has a need for them.
This can be interpreted in different ways. It could mean that the universe is sentient in some way, that the Guardians are a natural part of the order of things. It could also mean that they are self appointed guardians and they just believe that the universe needs them.
Indeed, for all the White Guardian’s ascertains that they will each exist eternally, they seem motivated to extinguish the other. The Black guardian is the champion of evil and chaos while the White Guardian is the champion of good and order.
What is apparent is that there do seem to be limits to what they can do. They both work through agents, manipulating lower races. It could be that their victory will mean nothing if they achieve it directly but they can ‘push’ others into following their agenda.
It is unclear if the prize for winning the race was even real. The Eternals believe that it will give them ultimate knowledge, freeing them from their dependence on other races. The Black Guardian taunts his counter-part that the Eternals are lacking morality and with the power bestowed upon them the universe will descend into chaos, which is his goal.
Upon Turlough rejecting the prize and destroying the Black Guardian the Doctor muses that Enlightenment was the choice whether to keep the prize or not. The peculiar thing is that all the Eternals had clearly made the choice to take the prize since they were all racing with that very aim.
If we assume then that Enlightenment would have given the Eternals the knowledge they sought it leads us to question how this scenario came about. At the prize giving the White and Black Guardian appear together, as if this a joint decision between the two of them.
If any of the Eternals had won it is hard to see what the White Guardian would have gained from bestowing the gift if, as the Black Guardian says, any one of them would have brought chaos.
It might be that the whole race was the Black Guardian’s plan but then if he had the power to bestow ultimate knowledge without the White Guardian why didn’t he give it straight to Captain Wrack, since he clearly favoured her.
The whole adventure just about works if the White Guardian was aware that Turlough was being manipulated by the Black Guardian into killing the Doctor. The death of the Doctor would rob the White Guardian of a valuable agent so he may have proposed the race to engineer the specific scenario in which Turlough can free himself. The Black Guardian could hardly refuse, having everything to gain and believing that having Turlough in his pocket tipped things in his favour.
Following this reasoning the player characters might find themselves caught up in the battles between the White and Black Guardians, performing little morality plays. Not only will they need to complete the quests assigned them they must also keep their souls from being corrupted by the Black Guardian. Who knows what he might tempt them with, as a group or individually.
Just as with Turlough a player character might find themselves facing certain death and be offered a second chance at life by the Black Guardian, only to find themselves eternally in his debt. This would certainly make a good Dark Secret.
It should be noted that the great power of the Guardians seem limited to communicating with others in order to deliver their messages. Turlough was occasionally physically choked by the Black Guardian but his main form of punishment was telepathic taunts.
The story begins with the White Guardian struggling to send a message to the Doctor and grows weaker as the story progresses, apparently because the forces of good are failing. It is not hard to imagine that the Black Guardian could similarly be weakened, in places where order and goodness hold reign.
In certain scenarios it is possible to imagine that the player character might actually find themselves working with the Black Guardian. Chaos is not always linked to evil, especially when the alternative is stagnant order, such as one imposed by a dictatorship.
Rebellious player character might find it enjoyable to become agents of change, travelling from one place to another at the Black Guardians behest. Could a campaign of positive chaos actually change the nature of the Black Guardian, no longer linking him to evil acts?
Using the Guardians in any form gives a campaign more structure, elevating the player characters to champions of the universe. If the player characters enjoy being free agents it is simple enough to end their contact with the Guardians by simply having their quest come to an end, as shown in the Key to Time.
After they are introduced the player characters will always know that there are greater forces at working universe and wonder when they will next become involved in their affairs.