Tears are replaced by a jaw bone melee weapon that is similar to The Forgotten's bone club.
The jaw bone deals 3× Isaac's damage when swung and 1.5× Isaac's damage per tick when thrown (3× Isaac's damage per second).
The swung jaw bone reflects enemy shots.
Reflected projectiles deal 6 damage if they would've hurt Isaac for half a heart, and 7 for a full heart.
The thrown jaw bone charges very quickly compared to the Forgotten's bone club, but as berserk only lasts five seconds, it uses a noticeable amount of the effect's limited duration to charge and throw the jaw bone.
While Berserking, Isaac's fire rate is halved then increased by 2. This will be a tears upgrade unless Isaac already has a lot of tears upgrades.
If playing as The Forgotten, the ×0.5 fire rate modifier isn't applied, as he already has it.
Killing an enemy while "Berserking" increases Berserk's current duration by 1 second.
Killing an enemy grants very brief invincibility, preventing damage from exploding enemies.
Every enemy killed will also grant +0.4 speed until the effect ends.
Isaac is unable to die while Berserk is active. He can be reduced to 0 hearts, but will not die until the effect ends.
This does not apply to insta-kill effects (such as Damocles and Plan C) - they will kill Isaac even if Berserk is active.
This item charges when Isaac deals damage to enemies rather than by clearing rooms or picking up batteries. It requires a base total of 120 damage, and increases by 40 for every next floor.
Notes
The jaw bone has all the same synergies that The Forgotten's bone club does.
Mom's Knife: While the synergy between Mom's Knife and the Berserk state is identical to The Forgotten's melee attack, it is worth noting that the attack no longer deflects enemy shots.
Synergies
Book of Virtues: Spawns a pale wisp on the middle ring. The wisp has extra HP and does not shoot.
C Section: Replaces club throw with two swings in quick succession which fire a fetus. All fetuses fired will hold bone clubs, similar to the interaction with The Forgotten.
Suplex!: When used near the end of the Berserk state (any time after the second bell ring), Suplex receives a unique visual effect, and will deal massive damage to the target, while setting off a Mama Mega! style explosion, destroying all obstacles in the current room and blowing open all doors. The target being Suplexed, as well as nearby enemies, will receive 300 base damage that scales with Isaac's size. All other enemies receive 200 flat damage.
The Lost / Tainted Lost: This can allow either variant of The Lost to survive being damaged if it happens while Berserk is active. Despite having no visible health bar, their health is normally set to half a soul heart. If they take damage while Berserk is active, they will not die if they are able to pick up a soul heart or half soul heart before the effect ends. Black hearts do not count for this.
The Ludovico Technique: While under the Berserk state, you lose control of the Ludovico Technique tear, which can now be deflected by the jaw bone swing, bouncing off nearby walls and dealing damage to enemies.
Ace Cards: Turning enemies into pickups doesn’t count as damaging or killing them for the sake of charging this item or extending its duration.
Car Battery: Duration is only extended if no enemies are killed during the first half, thus only providing a benefit against bosses that can't spawn enemies.
E. Coli / Flush!: Turning enemies into poop doesn’t count as damaging or killing them for the sake of charging this item or extending its duration.
Freeze Effects: Killing enemies by freezing them will not extend the timer, which can make it difficult to maintain the duration.
Libra: Because of Berserk giving you +0.4 speed with every kill, Libra will instead steadily increases the damage, tears, and range stats with seemingly no limit, allowing massively high DPS, but targets are much more difficult to hit due to the range of the swing gradually extending outwards more and more with each kill.
Sharp Plug: Isaac will take damage without gaining any charges.
This item is the active item equivalent of Tainted Samson's passive ability, with the only differences being that the player can choose to activate this, and this doesn't charge upon taking damage.
The item's name, description, and effects are likely a reference to the Berserk power-up from the Doom franchise, which forces the player to use powered-up melee attacks in some games.
The phrase 'rip and tear' is frequently used throughout the Doom franchise, and was first used in the 1996 Doom comic after protagonist Doomguy picked up the Berserk power-up.
In addition, Isaac's inability to die during the effect could be a reference to the common joke of Doomguy being "literally too angry to die", as well as his indestructible nature in the later series lore.
This item may also be a reference to the character Chicken from Nuclear Throne who, unlike other characters in the game, wields a katana and, upon losing all HP, instead of immediately dying, her head gets cut off and she can restore health in a short amount of time to stay alive before dying for good.
This item may also be a reference to the "Rage Beyond Death" ability with similar effects that Barbarians can gain in 5th edition of Dungeons & Dragons, as the original Isaac characters were modeled after character classes from the game.
The jawbone Isaac uses is a Donkey's jawbone, which is a reference to a passage from the Book of Judges in which Samson kills one thousand Philistines using only the jawbone of a donkey.
It may also be a reference to the item Donkey's Jawbone from Antibirth that didn't make the cut for Repentance.
This event is also referred to by the pickup message for the Soul of Samson — "Slay a thousand".
This item's special synergy with Suplex! was inspired by a bug in Repentance's initial release where any enemy (including final bosses like Hush and Delirium) that Suplex! was used on as Berserk!'s timer was ending would be instantly killed.
The Forgotten's bone club.
Mom's Knife: While the synergy between Mom's Knife and the Berserk state is identical to The Forgotten's melee attack, it is worth noting that the attack no longer deflects enemy shots.
Book of Virtues: Spawns a pale wisp on the middle ring. The wisp has extra HP and does not shoot.
C Section: Replaces club throw with two swings in quick succession which fire a fetus. All fetuses fired will hold bone clubs, similar to the interaction with 
Suplex!: When used near the end of the Berserk state (any time after the second bell ring), Suplex receives a unique visual effect, and will deal massive damage to the target, while setting off a 
Mama Mega! style explosion, destroying all obstacles in the current room and blowing open all doors. The target being Suplexed, as well as nearby enemies, will receive 300 base damage that scales with Isaac's size. All other enemies receive 200 flat damage.
The Lost / 
Tainted Lost: This can allow either variant of The Lost to survive being damaged if it happens while Berserk is active. Despite having no visible health bar, their health is normally set to half a soul heart. If they take damage while Berserk is active, they will not die if they are able to pick up a soul heart or half soul heart before the effect ends. Black hearts do not count for this.
The Ludovico Technique: While under the Berserk state, you lose control of the Ludovico Technique tear, which can now be deflected by the jaw bone swing, bouncing off nearby walls and dealing damage to enemies.
4.5 Volt / 
9 Volt / 
Jumper Cables: No effect.
Car Battery: Duration is only extended if no enemies are killed during the first half, thus only providing a benefit against bosses that can't spawn enemies.
E. Coli / 
Flush!: Turning enemies into poop doesn’t count as damaging or killing them for the sake of charging this item or extending its duration.
Libra: Because of Berserk giving you +0.4 speed with every kill, Libra will instead steadily increases the damage, tears, and range stats with seemingly no limit, allowing massively high DPS, but targets are much more difficult to hit due to the range of the swing gradually extending outwards more and more with each kill.
Sharp Plug: Isaac will take damage without gaining any charges.
Tainted Samson's passive ability, with the only differences being that the player can choose to activate this, and this doesn't charge upon taking damage.
Soul of Samson — "Slay a thousand".
Hush and 
Delirium) that Suplex! was used on as Berserk!'s timer was ending would be instantly killed.
Tainted Samson Unlocks 
Delirium Unlocks 
The Binding of Isaac: Rebirth
