What in the world is an arena death match?

[Amhazair]

What no one explicitly stated yet: The turn before that you saw all those battle reports of the arena, you'll have received a message telling you there would be an arena combat next month and that your chosen hero 'should depart now in order to reach the event in time' or something along those lines. For that turn only, all your commanders will have the order available to enter the arena competition.