Does Assault apply when attacking an open battlefield, and if it does apply when attacking an occupied battlefield, can the opponent use Gust (which targets units with 3 might or less) on a Vayne with base 2 might and Assault 3 during the showdown?

Ruling: Assault does not apply when attacking an open battlefield because there are no attackers or defenders in an open showdown. When attacking an occupied battlefield, Assault applies as soon as the unit is designated as an attacker, so Vayne would have 5 might and cannot be targeted by Gust.

Sequence:

• When attacking an occupied battlefield, units are designated as attackers
• Assault applies during a specific step of the showdown start
• The might increase from Assault counts during the showdown
• Gust checks the current might of the unit, which includes the Assault bonus

Nuances:

• Assault is a passive/continuous ability that applies (not activates or triggers) when conditions are met
• In open showdowns (attacking open battlefields), there are no attackers or defenders, so Assault does not apply