Revenge Mode is a gameplay feature introduced in Disgaea 5: Alliance of Vengeance. During battle, when an ally unit receives damage or is defeated, the Revenge Gauge automatically fills up. When it reaches to max, Revenge Mode is activated. Once Revenge mode is active many various stats increases.
When you receive attacks from the enemy, your Revenge Gauge will increase, which is the meter to the left of your character's face in the bottom left. Once it has been completely filled, that character will automatically enter Revenge Mode, which has three basic benefits. First, the character's Critical Rate will now be 100%; the SP cost of skills is also reduced to 1 and you will take 25% less damage. The Revenge Meter will also fill up if your allies are killed and whenever you defeat enemies that are in Revenge Mode. This mode only lasts for three full turns before that character will return to normal. Some characters, though, have one more advantage, as they can learn a special skill called an Overload. This skill is only usable during Revenge Mode and can be used once per battle. Each Overload has a different effect. Enemies can also enter revenge mode and gain the same benefits, including using an overload power if they have one. But if you defeat an enemy that is in revenge mode, you will gain a revenge shard, which can be used to increase certain stats.