Berserker mode is unlocked relatively early in the game and it makes you much more powerfull when activated. Press
to activate, you will see that your claws glow red, as long as they are red Berserker mode is active. You can only activate Berserker mode when you have enough Rage (the yellow bar below the health bar represents the amount of Rage you have). Kill a total of 150 enemies while Berserker mode is active. It's best to use in areas with many enemies, as it uses a lot of your rage meter. Also, equiping the Inner Rage Mutagen helps you get more Rage from kills, refilling your Rage meter faster, so you can activate Berserker mode more often.