You can only get the Sword of Kings before defeating the boss of the Stonehenge base. All the monsters in the area vanish once the boss is slain, and the weapon can no longer be obtained. That said, Poo is plenty strong without it - None of the super rare weapons/armour pieces are necessary to complete the game.

it’s also a FUCK to grind, absolutely doable but you gotta wiggle the grinding method. i always get it but i’m not sure it’s worth it lol