then childe found out tsaritsa going to make teucer enter x-10 experiment to develop the next generation humanoid combat weapons that can fight against Houkai. childe volunteer himself instead to protect his brother. the experiment failed and he found himself in sea of quanta

lost in the sea of quanta. childe accidentally found the 2nd herrscher's core and somehow it absorb to his body and they become one. childe gain stigmata (in riptide shape) and he starting to noticed the second soul living in his body, tartaglia

tartaglia won't stop talking with him, sometime try to take over his body. but the two still stuck in the sea of quanta. lost in bubble universe. untill scaramouche come out of nowhere and drag childe out of this place.

with a help of skirk (which she also ask help from venti who have higher rank and have more power) scaramouche can bring childe back from sea of quanta. and those two enter st. barbatos learning to be vakyrie and leave under venti's wings instead

tartaglia become to take favourite in childe. but he would always seek a chance to take over the body. his hatred for humanity never gone the only exception is childe. (he's a mix of veliona and sirin, yes)