At a battlefield my opponent has a Moto Chagatai Exp 5 in his army and only Unicorns. I take my action to destroy one of my Tacticians with Soul of Battle. Soul of Battle' effect adds the additional cost of bowing to my opponent's next action. Moto Chagatai negates the first time another player's card effect bows a Unicorn personality at his location.
Would the additional cost added by my Soul of Battle's effect be negated by Chagatai's trait if he tries to pay it?
In terms of the good faith rule you must be able to pay all costs of an action you are aware of in order to take an action. If my opponent is unable to bow a personality because of Chagatai's trait (or any other effect the prevents bowing) would that mean he is unable to take an action for the remainder of the battle?
Soul of Battle: "Battle: Destroy one of your opposed target Tacticians or Deathseekers in the current battlefield: Gain 3 Honor. The next action the enemy leader announces this battle has the additional cost "Bow one of your Personalities at the current battlefield."
Moto Chagatai Exp 5's Trait "Before the first time each phase that another player's card's effect bows a Unicorn Clan Personality at Chagatai's location: Negate the bowing."
Good Faith Rule: To legally announce an action or play a card, you must first be able to do it in “good faith”; that is, its requirements must be met, you must be able to pay its costs, and you must be able to meet any required targets in an action’s targeting step. For actions that attach a card, you must be able to pay the card’s modified costs and attach it legally...Aspects of the game state should be considered under the good faith rule, such as effects that reduce costs, prevent costs from being paid, or relax requirements.