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At the end of the mermaid arc, even though Luxia thought she won and left the world, the villain who was dying, was actually waiting for the Superior Demon System which he learned about from Bamboo Rattie. The Superior Demon System was from the "CEO wishes to go on strike" arc.
730+ chs, but the chapters are so short it really feels more like 300. Anyway, spoilers ahead...!
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The arc after this one (Jing Si), is the Greek God arc, and we finally get a little crumb of what happened to Zero. Starts around 440s.
Gonna skip mentioning a bunch of arcs but the most notable for me were:
Around the early 600s is the Mermaid Arc, which was really exciting. The villain made my jaw drop at the end because of a certain someone's cameo. I really worry for Luxia in the future...
Around the mid 600s, is the Evil Taoist Master arc, which is directly related to Zero. (Directly after this arc is the return of the female Taoist girl from a few arcs ago.)
Luxia also gets to hang out with the male agent from the spin-off for 1ch around the 730s. They've confirmed the locations of Zero's fragments.