If Volibear, Furious moves to an empty battlefield to conquer it, and during the showdown an opponent uses a spell to move a unit there (starting combat), does Volibear's on-attack trigger activate?

Ruling: Yes, Volibear's on-attack trigger happens when the combat begins. When the first showdown ends, a new showdown immediately begins as a combat showdown, and since Volibear contested the battlefield first, his controller and their units are considered attackers, triggering the on-attack ability.

Sequence:

• Volibear moves to empty battlefield, starting a showdown
• Opponent uses spell to move a unit to that battlefield during the showdown
• First showdown ends
• New showdown immediately begins as a combat showdown
• Volibear's controller is the attacker
• Initial chain happens normally, including Volibear's on-attack trigger

Nuances:

• The first showdown does not assign any attackers or defenders since it was not opened as a combat showdown, so effects like assault or shield do not apply during that first showdown