The Excalibur, similar to the Machete, is a replacement for the whip that not only deals more damage, but can also hit stunned enemies as well, due to the Excalibur's high damage however, this is usually not seen unless the enemy is stunned some other way. Furthermore, it is also able to damage Kingu's shell, which is integral to obtaining the Tablet of Destiny. If the Excalibur is hit by the Clone Gun, it will instead produce a Broken Sword.
The Excalibur, as well as the method of obtaining it, is a reference to the sword of the same name from Arthurian legend. Another sword, often conflated with the Excalibur, was pulled from a stone by King Arthur, as proof of his royal lineage.