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Davis and Evelynn left the rift space they made fond memories in, but before leaving, they didn't forget to take the poison essence with them, which was the clear spring. Such pure poison essence was hard to come by, which made Evelynn a bit happier because she could improve her Poison Laws with this to a great degree.
It could've also produced its own Spirit Attribute Source if it happened to experience a suitable reaction.
Still, according to Mival Silverwind and the records she read from the Myriad Poison Sect, also obtaining some suitable manuals, she knew that she should be able to create a Rune and a Crest. Then, once she reaches the peak of Level Six Abstruse Intent in Poison Laws, she could even try making an Imposing Rune after perfecting her domain and law sea that she never got to complete as her body turned into a fey.