With the creation of many new berserks, we decided they deserved their own page. Berserks help Barbarians in dangerous situations that might otherwise kill them. That said, not all of them will aid in escaping combat as the old single berserk ability did. They also last longer than the old berserk did, but you cannot bring them to an end prematurely. To see what berserks are available to you, type berserk help.

Notes
Skiori (I should hope you know who this is) has said, "Strength, agility, and stamina all improve the power of a berserk. Discipline lowers the power." Discipline lowers the duration of a berserk, but not its strength. This should only be a problem if your discipline is far above your other stats.
The berserk called Cyclone is the last that is taught by guild leaders. All of the higher berserks can only be learned through quests.
To differing degrees, all berserks effectively decrease intelligence, but they all boost BMR.
Berserks may prevent you from using some verbs in game, such as look and retreat.
Berserks lower inner fire. Their duration is based on circle and stats.
Inner fire level and stacking berserks lessens their duration as does spending time out of combat while berserking.
The penalty received when a berserk ends is determined by how low your inner fire is at the end. The penalties include stuns, fatigue and falling over.

Blood can be learned at 2nd Circle and boosts strength and offense.
Example: Celestiaa explodes in a violent whirlwind of dangerous power, her bloodshot eyes gleaming with an insatiable lust for battle!

Stone can be learned at 5th Circle. This berserk boosts defenses and vitality.
Example: Celestiaa snarls viciously, her teeth bared in hunger that is decidedly short of sanity!

Nightmare, gained at 12th Circle, boosts one's intimidation factor, aiding with roars. This also boosts strength and defense against intimidation.
Example: Growling like a dog latched onto a fresh kill, Celestiaa's eyes dart about with the intensity of a ruthless predator!

Flame, gained at 20th, boosts strength and melee offensive skills more than any other low level berserk, but is relatively short in duration. This may boost accuracy.
Example: Celestiaa's face contorts into a horrific rictus of a smile as she lunges forward!

Cyclone is gained at 35th Circle and boosts speed, balance, and agility.
Example: Surging forward with a howl of mad glee, Celestiaa grins wildly at some disturbed thought.

Steel is gained at 50th Circle and boosts health and defense.
Example: A snarl of insurmountable arrogance erupts from Celestiaa as her eyes widen in maddened battle-lust!
Note: Skiori has said, "This berserk grants a slightly better MR boost than Dragon does."

Grave is gained at 70th Circle and makes you stronger through killing.

The ability to break a dance by berserking is gained at 80th Circle.

Kuniyo is gained at 100th Circle and gives a large boost to offense.

Everild is gained at 120th Circle and boosts offense.

Trothfang is gained at 150th Circle and boosts offense.

The ability to berserk while continuing a dance is gained at 180th Circle.

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We would like to thank Caraamon, Niamara and Maulem for much of this information. In addition, we thank Celestiaa for her useful post in the DR Forums of how berserks appear to others as well as everyone else who posted useful information there.