King Pirate's Sword

Bonuses:
Level 1: +3 Physical Strength
Level 2: Courage (Friendly creatures are immune to harmful effects on initiative.)
Level 3: +5 Leadership
Level 4: Preparation for Boarding (Friendly creatures gain +10 to morale, +5 to initiative, and +2 to movement range on the first turn of battle.)
Level 5: Free Rum (Increases the morale of all friendly creatures by 15. Duration: 3 turns. Charges: 1.)
Added 59 minutes later
As an addition, here's another unique "weapon" - Companions:
These are the companions of the main heroes on the adventure map. They accompany him, help in battle, or in scouting.
So far, only two Companions are known - Kreg Hack and Sandro.
Perhaps more will appear later.