The sword of kings in the Earthbound player's guide.
The Sword of kings is a weapon found in EarthBound. It is an extremely rare item, having a 1 in 128 (0.78125%) chance of being dropped by a Starman Super in the Stonehenge Base. The weapon's rarity is directly related to its power, as the Sword of kings is the only weapon that Poo can equip that increase his attack power.
Trivia
Equipping the Casey Bat may make it somewhat easier to get the Sword of Kings due to a higher chance of scoring Instant Wins.