Fist of Darkness Blox Fruits: Obtaining

Fist of Darkness is the emblematic quest item of the Second Sea, the exact counterpart to God's Chalice in the Third Sea. It serves to summon Darkbeard, to initiate the quest chain for the Core Brain for the Cyborg race, and in duplicate, to obtain the Slayer skin for the Dark Blade. It's also incredibly fragile - impossible to store, impossible to drop, and it disappears as soon as you die or leave the game.

How to get it

Defeat a Sea Beast: approximately 2.5 percent chance, and this is the method recommended by the Fandom, because a Sea Beast spawns and farms much faster than a chest cycle.
Open a chest: four hours after a server launches, a Fist of Darkness appears in a random chest. The probability therefore depends on the time elapsed since the server started, which you can check in the menu.

The very first time you obtain one from a chest, you unlock the title Forbidden One (number 116).

Its three uses

Summon Darkbeard at Dark Arena.
Insert it at the center of the Secret Laboratory in Hot and Cold: you then unlock a chance to obtain a Core Brain from Order, the essential piece for the Cyborg race.
Obtain the Slayer skin for the Dark Blade, a reskin of the original weapon. You need two: one for Darkbeard, one for the Graveyard Door, in addition to Angel, Rabbit, Shark, and Human in V3 or better.

Key rules to know

Impossible to store in Inventory.
It disappears if you die or leave the game.
Impossible to drop or give away.
Only one at a time: picking up a second one produces absolutely nothing.
However, the insertion at the laboratory is saved. Even after disconnecting, you can always claim your Core Brain from Order.
A chest that has already appeared without a Fist of Darkness will never contain one later.

Farming tips

Like with God's Chalice, the real challenge isn't obtaining it but keeping it until you use it.

Avoid public servers: someone may have already emptied the chests, and a bounty hunter who kills you will make you lose it. A private server solves the problem, otherwise wait in Safe Zone until you're ready.
For chest hunting, Cursed Ship and Café are the best spots, because they concentrate many chests in the same location. You collect everything, then switch servers.
The Portal fruit, especially in permanent version, makes this route much faster if you know the chest locations in the Second Sea.
Still, the Fandom discourages the chest method because of the four-hour cycle and instead recommends hunting Sea Beasts.

Details

In-game, the object simply displays as Fist of Darkness with no icon, despite the image shown on the Fandom.
Fist of Darkness and God's Chalice fill exactly the same role in their respective seas: a rare artifact, dropped by mini-bosses or found in chests, that serves to summon a Raid Boss.