Vengeance is an attack style characterized by stats or gameplay events that dynamically change upon losing or gaining lives. It is introduced in Bloons TD 6 with the introduction of Heart of Vengeance. It is part of the attack interactions family.
Vengeance is represented on the Bloons Wiki as , the upgrade icon for Heart of Vengeance in Bloons TD 6, since that is one of the only upgrades that does this. It is different from Retaliation in that Vengeance reacts based on how many lives the player currently has and that the effect is permanent (barring any changes in lives) rather than an ability that has a cooldown.
For Heart of Vengeance's case, Vengeance gains a stronger attack speed buff the more lives that are lost, and a weaker (and maybe nonexistent) attack speed buff the more lives that are regained. Vengeance is also given to Heart of Oak since Version 39.0, with the upgrade getting more pierce for its attacks the more extra lives the player has, and the extra pierce can be lost if those extra lives are lost. If there is any attack that does not increase or decrease in stats, depending on the number of lives the player has, then it should not be considered Vengeance .
All attacks require an upgrade in order to have Vengeance.