What happened to Xíng Hài?
Xíng Hài is alive. she is hinted at in the second part to Seishi Ikken as being the one who the Xuán Guǐ Zōng remnants contacted who made it so that when they die their blood would turn into acid (a spell Xíng Hài mentioned in the first season) 

However she makes a proper appearance after the credits of the final episode of Season 2 where Huò Shì Míng Huáng, talking about Xiē Yīngluò's failure to get the Sorcerous Sword Index from Shāng Bù Huàn and how the organization may need to lead an invasion into Dong-Li to get it. it is then that Xíng Hài appears stating she is a messager from the demon world and proposing that Huò Shì Míng Huáng's organization accepts the assistance of the Demonic Armies for their invasion