Going into a berserker frenzy is an innate, passive ability of the warrior class that is present from level 1 onward. Anytime a player character warrior's health falls too low, he or she enters a berserker frenzy, gaining a small bonus to attack rating and converting all of his or her critical hits into more powerful crippling blows.
It is not to be confused with the berserker class's Frenzy skill, which is something else entirely.
Detailed Mechanics[]
A warrior will go berserk when falling below 35% health and will remain berserk permanently until his or her health reaches 45%. (Need confirmation of numbers.)
While berserk, the warrior gains a small amount of attack rating (how much?) and will score a crippling blow whenever he or she would normally have scored a critical hit.
Going berserk is accompanied by the message, "Soandso goes into a berserker frenzy!" which will appear in the chat boxes of both the warrior and another other nearby players. "Soandso is no longer berserk" signals the end of the effect.
Berserking is a hard-coded mechanic of the class and stacks with all buffs, disciplines, etc.
Strategy[]
Berserking passively helps low-level warriors muster enough damage to finish fights when running dangerously low on health. In certain situations, the berserk state can be more deliberately leveraged to increase a warrior's DPS, even at high levels.
Increasing DPS while Berserk[]
A warrior of level 54+ can combine the berserk state with Mighty Strike Discipline to turn every attack (for up to thirty seconds) into a crippling blow, resulting in an impresive burn if the warrior remains low on health for the duration.
More generally, any group buffs that increase crit chance (e.g., the shaman's epic 2.0 clicky) will help a warrior deal more damage while berserk. From the level 67 "Speed of Salik" spell onward, enchanter haste also increases crit chance, making it very desirable for warriors who want to milk their berserking for DPS.
Tricks for Entering Berserk Safely[]
In a party in which someoene else is doing all of the tanking, a warrior can /shield the main tank as a way to manipulate his or her own health down into the berserking range, thereby gaining consistent access to crippling blows, which help the warrior perform better in this off-DPS role. (Note that this only works in parties with player healers, since mercenary healers will heal the warrior regardless.) In the event that the warrior may end up needing to tank an add after all, having an ability like Fortitude Discipline or Furious Discipline at the ready is advisable, in order to give the healer(s) time to heal.
Manipulating the warrior's maximum health while not changing his/her current health can also aid in reaching the berserk state at a desired time. For instance, a druid or shaman of level 81+ can use the Wild Growth line of spells to give the warrior a large temporary buff to max health. This buff does not increase the warrior's current health at all, causing the warrior's percentage of current health percentage to experience a sudden, artificial dip. If this change reduces the warrior's current health below 30%, the warrior will go berserk the next time he/she takes damage. (Note that the warrior's Warlord's Tenacity AA does not work for this purpose, since it also heals the warrior upon use.)
Underhealing to Maintain Berserk Status[]
Generally speaking, it is also possible for a warrior and priest class duo to keep a warrior permanently in the berserk range anytime the pair are facing enemies that are sufficiently trivial that there is no danger of them killing the warrior quickly. Since warrior + priest duos tend to have low DPS, keeping the warrior berserk (for the chance at crippling blows) may be desirable as a way of speeding up clear times.
Shamans make particularly easy partners for this, since their attack speed slows and HoTs make it hard for trivial opponents to muster enough DPS to bring the warrior down, while also allowing for predictable healing that doesn't bring the warrior out of the berserk state. (The warrior can click off the shaman's HoT if an upcoming tick of the heal is foreseeably going to end the berserking.)
Similar results can be achieved with a cleric who maintains a Vie buff on the warrior at all times and provides a HoT to keep the warrior hovering between 30% and 45% once the initial berserk state has been reached.
Druids participating in this tactic will want to focus on carefully timed patch heals, Adrenaline heals (with their Vie-like proc chance), Reptile healing, and (at 85+) other predictable reactive heals like Surge of Bonebriars (and other later spells in the Surge of Ironvines spell line. Debuffing mob attack ratings can also be helpful.
In the absence of a player healer, a well-timed Distillate of Celestial Healing may also be able to get the job done, depending on just how trivial the foes in question really are. A warrior soloing highly trivial foes with a cleric merc may set the merc to passive long enough to fall below 30% health, use a strong Distillate of Celestial Healing, and make use of Mighty Strike.
If a warrior and a priest class are being leveled up together, with the intent to be played primarily as a duo, the priest might consider lowering the priority of Healing Gift AA investment early on, since its healing crits cannot be turned off and are totally random, making it more unreliable to use patch heals when trying to underheal the warrior without him/her actually dying. Healing Adept is a good alternative, since the healing bonus it provides is a consistent percentage.