Thunder Sword is a combo ability available in Kirby: Squeak Squad. Kirby gets this by combining the Spark and Sword abilities. It is similar to the regular Sword ability but has more range and it has the element of electricity equipped with it, meaning it can send sparks over water and electrocute/slash enemies at the same time. It turns into a normal sword underwater, but when Kirby jumps out of the water, it turns back into an electrified sword.
Kirby slashes the enemy with his sword.
B + B
Kirby swings his sword very fast in slashes at different angles.
B + B + B
Kirby attacks with lightning-fast sword slashes that are nearly impossible to see.
Up + B
Kirby jumps in the air and does a flip, then he falls sword outstretched. This move comes from Cutter.
Kirby swings the sword quickly in midair, damaging all enemies that come into contact with it.
Curiously, along with the Fire Sword and Ice Sword abilities, the flavor text says it can't be used in water, when the truth is that it turns into a normal sword underwater. It most likely means that its electrical properties cannot be used underwater.
It is worth noting that, although Fire Sword and Ice Sword retain the names of their original Copy Abilities, (Fire and Ice, respectively), this Copy Ability opted for the name "Thunder Sword" rather than "Spark Sword."