code:

15 = {
        trigger = {
                event_target:enemy_commander = {
                        is_alive = yes
                        not = { has_character_flag = about_to_challenge_enemy_commander } #so this isn't about to fire the other way around
                }
                not = { has_character_modifier = battle_events_4_cooldown }
                nand = {
                        not = { has_pet_animal_trigger = yes } #the event is purely negative without the pet to sacrifice
                        is_indomitable_trigger = yes
                }
        }
        bad_soldier_outcome_score = yes
        warrior_lodge_risk_reduction_score = yes

        additive_modifier = { #for the more varied versions...
                has_pet_animal_trigger = yes
                value = 5
        }

        add_character_modifier = { name = battle_events_4_cooldown duration = 365 hidden = yes }
        set_character_flag = about_to_challenge_enemy_commander

        character_event = { id = hfp.11025 } # your pet to the rescue (otherwise something else)!
}