The Trial Sword (試しの剣, Tameshi No Ken, translates as Sword of Trials; The Trial Sword in the English Dub) is a weapon that Suzuki made from a tree in Spirit World. It is a prototype version of Shishiwakamaru's Banshee Shriek Sword. Kuwabara called it his "Signature Special Sword" (必殺スペシャルソード, Hissatsu Special Sword) when Suzuki gave it to him.
The Trial Sword is a normal looking black and light gray sword handle. When used by Shishiwakamaru it can be opened from the middle revealing a fleshy skull. There's no noticeable change when in use of Kuwabara, only forming a multicolored (as a sword) or magenta (as a flyswatter) spiritual energy blade similar to his Spirit Sword. Appearance may differ from user to user.
The Trial Sword and the Banshee Shriek Sword has the ability to absorb and enhance the energy of its wielder to its possible fullest at the time it is used. The prototype Trial Sword has been mentioned by Kuwabara to be the reason why his powers were gone - which was actually evolving inside him silently into the Dimension Sword.
Dark Tournament Saga
Suzuki gives this sword to Shishiwakamaru, where it became the Banshee Shriek, but was broken in two when Shishiwakamaru was defeated by Genkai. Suzuki made a new one and he gave it to Kazuma Kuwabara.
Kuwabara uses the Trial Sword to defeat Elder Toguro.
Chapter Black Saga
However, Kuwabara mentions it early on during the Chapter Black saga, when he thinks that the reason that his powers were disappearing was a side-effect of the Trial Sword.
Three Kings Saga
It is unknown if Shishiwakamaru obtained Kuwabara's old Trial Sword or if Suzuki made a new one for him, but he was seen using his own Trial Sword (Banshee Shriek Sword) during the Makai Tournament.