He traded his soul for immense power. A single strike from his sword slaughters legions of enemies. And the closer this brutal Slayer subclass comes to death, the deadlier he becomes. Yielding to demonic possession, this Slayer taps into an otherworldly rage and becomes a Berserker, capable of a wide range of vicious attacks, including Frenzy Mode, which unleashes furious bloodshed on threatening foes (From Nexon website)
The Berserker is a Slayer subclass. Berserkers are reliant on heavy damage, hit point based skills, and the "bloodied" status. They function optimally when near death, so most of their skills are based on losing life for more power. They can leech life from enemies, using bloodlust, or they can allow frenzy to drain their life to the near-end and then allow bloody cross to boost their strength and evasion rate.
They awaken as a Hellbringer.