One of the contestants is a defiler, as has come into the knowledge of the archdruid, who is supervising the combats. He doesn’t know which of the four, though.
A short word on defiling: a defiler is a magic-user, who draws his powers from the forces of nature. Thus, the more potent the spell he uses, the greater the effects on his surrounding nature are (leaves turning brown, twigs falling off, ... ). Naturally, druids and defilers don’t get along very well. However, if there’s no druid to a region, a defiler could carry out his experiments to a threatening degree.
Also, note that the contestants will try to use this situation to gain advantage over the candidate they’re most afraid of, by setting rumours that will point to that candidate being the defiler.
Moreover, the initiates will not be very cooperative, claiming that they need all their powers for the final contest that’s going to come up once the defiler is found.
With my group, it was very easy hooking them onto this, since one of the characters managed to upset a coven of witches, that was attending the festival. They put a curse on him (also, there was a time limit), which none of the other people at the festival could lift but one of the druids or the arch-druid himself.

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