Singing and Dancing Sword - Companion, Weapon, or Upgrade?
It's a separate piece of equipment that does not use a slot, meaning you still have two hands free for whatever you want to do with them. The card does not depend on you having another weapon in any way.

The concept it represents is a weapon that can just kind of float next to you and attack stuff for you without needing you to swing it ("dancing swords" from D&amp;D and various stories).