The Tourney begins with the opponents are present within The Hub, a semi-extra-planar realm from which numerous other locations can be accessed though magical doorways.
Once both opponents and any allies they have with them, either from prior the start of the Tourney or from one of the locations they visit, enter the Place of Battle, only one is permitted to leave alive. Each champion is permitted to send one of their allies forward for a duel to the death, with the victor returning to their place of origin - however, once a champion has no more allies to call upon, they must kill each of their opponents in turn themselves.
The rules of the Tourney are incredibly strict. The first and foremost of these has already been mentioned - the fights are one against one and to the death. The second is that the ultimate victor may ask his foe one question which must be answered truthfully, or a single favour (within his remaining power) that must be given.
Individual allies summoned from the other regions accessible from The Hub are instinctively aware of any particular regulations applied to them - for example, the Ki-Rin sent against the Dai-Oni by the Samurai Kensei was only permitted to prevent it from using its magic in battle, and could not duel it in the conventional sense. Similarly, the Knights of the Golden Company are considered a single adversary by the rules of the Tourney, even though they may outnumber their foe 10:1 or more.