As an Outer Disciple, Shen Xiao originally did not possess such potent toxic liquids; however, he obtained this bottle from Elder Fu Du, whose toxic concoction filled him with pride and satisfaction.
After the first round of the Outer Sect Competition, Fu Du took the initiative to visit Shen Xiao and gave him this bottle of toxic liquid. Clearly, Fu Du's target was not the Ling Wan, but the youth named Yun Xiao.
Perhaps even Fu Du had not anticipated that Shen Xiao would use this bottle of toxic liquid during his fight with a chubby boy at the Late Stage of the Meridian Gathering Realm, which disrupted his plans somewhat.
However, Fu Du, being a secretive man, did not show any emotion in front of the crowd, and he even intended to disassociate himself from this bottle of toxic liquid.