Potion of the Berserker

Disclaimer: This concept is very complicated

The Potion of the Berserker Gives the Rage effect for the same amount of time as regeneration of the same level, making a player's entire health bar a metallic color. Any lost hearts will take the form of cracks in the new metal hearts.

When a player with the Rage effect is hit, it will crack the number of hearts it would have damaged the player by. When all the hearts have been cracked, the player's health bar will start violently shaking. Regenerating by natural means is impossible, but if one uses magic to heal themselves, the cracks will disappear. When all hearts have been cracked, the metal will start to fall off with further damage. The metal can be revived with magical healing. If all the metal falls off, the player will begin to take damage as normal.

The Potion of the Berserker II allows for 20 extra damage before the metal hearts finally fall off.

When the effect ends, remaining metal hearts will dissipate, and the player will take damage on their normal health bar equivalent to [(damage taken during effect x 0.8) - hearts that would have regenerated during effect], meaning that if they took 25 damage after healing is accounted for, then they will immediately die, with a new death message that says "[Player]'s injuries caught up to them."

Brewing
The Potion of the Berserker comes from adding a Golden Apple to a Potion of Resistance (Iron + Awkward Potion (Water + Nether Wart))