Jack_of_shadows
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For now, I only have one hypothesis. Before finally agreeing on the battle, there is an option in the dialogue to rehearse it. The rehearsal is not time-limited; it will continue until the hero lands 12 hits (normal attacks) on the dragon.
In the quest log, this stage is marked as "A suitable dragon was found in its lair...".
If you proceed to the next stage and set a specific target and attack time, the quest log will display the entry "The Black Dragon Dart agreed to stage your brave victory over him, after he...".