Pillars of Eternity II: Deadfire

First you need to find one weapon in the list and bind it to an appropriate character.

Next you need to follow the steps it specifies to unlock all the powers. The see the conditions to level up the weapon, you go on it in your inventory and press .

Personnaly, i've chosen Magistrate's Cudgel. It's really easy to level up.

Some weapons are really to level up, others are hard.

Soulbound items found in the base game:

Blightheart (arquebus) – found at Burial Mound south-west of Motare o Kōzi
Lord Darryn's Voulge (pollaxe) – found at Shipwreck south west from Takenaka in an enclose island
Magistrate's Cudgel (mace) – found at Subterranean Temple north of Poko Kohara.
Marux Amanth (dagger) – found at Poko Kohara Ruins (Engwithan Waystation, Level 2).
Modwyr (sword) – found at Old City Ruins, small hideout in the northeast part of the area.

Soulbound items in Beast of Winter DLC:

Guardian's Plate (breastplate) - found on a frozen elf by the gate in the Temple of Decline area.
Wintertide Bulwark (shield) - found in a tomb in the Eastern Cave in The Rimebound Temple area.

Soulbound items in Seeker, Slayer, Survivor DLC:

Seeker's Fang (rapier) - granted as a reward for winning The Contest of the Seeker challenge and becoming the chosen of the Seeking Face.
Slayer's Claw (battle axe) - granted as a reward for winning The Contest of the Slayer challenge and becoming the chosen of the Slaying Face.
Survivor's Tusks (helm) - granted as a reward for winning The Contest of the Survivor challenge and becoming the chosen of the Surviving Face.
The Changeling's Mantle (hide armor) - reward for Tactical Ogres challenge (challenge unlocked by the Luckless Coin artifact).

Soulbound items in The Forgotten Sanctum DLC:

Robes of the Weyc (cloth armor) - found on the Spun Terrace.
The Weyc's Wand (wand) - found in a chest the Quarantined Section of the Archives.