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
also note if you do decide to grind for the Sword of Kings, chances are you will gain dozens of levels just trying for a CHANCE to get the sword so if powerleveling is in any way not your thing, pay some heed
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
also note if you do decide to grind for the Sword of Kings, chances are you will gain dozens of levels just trying for a CHANCE to get the sword so if powerleveling is in any way not your thing, pay some heed