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.