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The Serpent's Tail was a Shen Gong Wu that made things intangible.
As the name implies, the Shen Gong Wu looks like tail of a Serpent, being long and flexible. It has a mustard yellow fins at the mid-range and end of the tail and has a yellow part under it.
When this Shen Gong Wu was activated, it allowed the user to travel through solid objects such as walls and allowed them to avoid physical attacks but it didn't protect them against non-physical attacks and could be taken away while in use.
When the Serpent's Tail was combined with the Reversing Mirror, it could turn someone who was ghostly into a flesh and blood person, as seen when it was done to Wuya twice in in the series.
The Serpent's Tail was found in New York, located in the subway underground, Omi and Jack Spicer grab the Serpent's Tail and challenges ease other to a Shen Yi Bu Dare for the Serpent's Tail in a basketball game, Omi had Jermaine as his partner while Jack had his robotic train become his partner. The Serpent's Tail turn itself into a basketball to be used in the game. Omi and Jermaine won the showdown along with the Serpent's Tail (turn itself back into it's normal form), Jetbootsu and the Shroud of Shadows (strangely was not seen with Omi).
Clay Bailey wagered the Serpent's Tail in a Xiaolin Showdown against Jack Spicer for the Star Hanabi in "Hanabi in a Haystack. Clay used the Serpent's Tail to pass through a giant haystack to found the Star Hanabi, but to no avail. Clay wins the Xiaolin Showdown along with the Star Hanabi and the Sword of the Storm.
Clay Bailed wagered the Serpent's Tail in another Xiaolin Showdown, but this time against Tubbimura for the Tongue of Saiping in "Race Across the Ice". When a giant ice wall appeared in front of them, Clay used the Serpent's Tail for his advantage and pass straight through the ice wall, Unfortunately, Clay loses the showdown as result, losing the Serpent's Tail. After Omi defeated the Heylin villains using the Sun Chi Lantern and saved Clay, Raimundo and Kimiko, The monks manage to obtain the Serpent's Tail along with the Longi Kite, Tongue of Saiping and the Orb of Tornami from Jack Spicer's hideout.
Upon the rise of Mala Mala Jong, The Xiaolin monks flee the temple grounds along carry the Shen Gong Wu with them, In a cave on Mount Hong, Clay suggesting to used the Serpent's Tail to remove the Heart of Jong form Mala Mala Jong located in the Two-Ton Tunic, but Kimiko remarks Clay that the Serpent's Tail wasn't be able to protect them from the lightning blast of the Eye of Dashi. Omi used the Serpent's Tail to travel underground and found the Golden Tiger Claws located in the enter of the Earth. Omi had many fail attempts, but managed to re-obtain the Golden Tiger Claws.
Did You Know...
When the Serpent's Tail is combine with the Reversing Mirror, it can actually bring someone back from the dead. This is because it can make an intangible ghost tangible.